How Do I Know If Malware Is On My Computer?

Updated on September 1, 2026, by Xcitium

You may have malware on your computer if it suddenly becomes slow, crashes frequently, displays unexpected pop-ups, redirects your browser, installs unknown programs, changes settings without permission, disables security software, uses unusual amounts of CPU or network bandwidth, or makes files inaccessible.

However, symptoms alone do not prove malware is present. Hardware problems, software conflicts, updates, and configuration errors can cause similar behavior. The easiest way to confirm a suspected infection is to run an updated antivirus or malware scan.

Cybercrime is at an all-time high. Criminals are not only targeting home users; they are also preying on small businesses jeopardizing their websites and acquiring sensitive clientele information. How malware infects your computer? Are you a potential victim? How can you protect your assets? In the article, learn the basic and best ways to find and remove malware.

What Does Malware Do?

Malware programs are dangerous threats to your system. The very reason that makes malware a potential threat is that it operates without your knowledge and may go undetected even by the antivirus installed on your computer.

Your computer may become infected with malware when you have unknowingly downloaded a free software or application that was available online. Malware comes in the form of viruses, Trojans, worms, adware, spyware, etc.

Some malware types such as spyware are capable of tracking your every move once it enters the system. It can also make your system inoperable, fill your computer screen with popup advertisements, slow down your computer, and delete files on your computer. When malware infects your computer, it adversely impacts your system performance.

15 Signs Malware May Be on Your Computer

Your computer suddenly becomes very slow.
Programs freeze or crash frequently.
The PC restarts unexpectedly.
You see persistent pop-ups or fake virus warnings.
Your browser redirects to unfamiliar websites.
Your homepage or search engine changes without permission.
Unknown applications or browser extensions appear.
CPU, memory, disk, or internet usage is unusually high.
Your antivirus becomes disabled or cannot update.
You cannot access security websites or system tools.
Files disappear, become corrupted, or receive strange extensions.
Your disk space suddenly decreases for no clear reason.
Emails or messages are sent from your accounts without you.
You see unfamiliar account logins or password-reset activity.
Your computer behaves strangely while you are not using it.

These symptoms align closely with current guidance from Malwarebytes, ESET, Norton, and the NCSC.

1. Your Computer Suddenly Runs Slowly

Malware can consume processing power, memory, disk activity, or internet bandwidth in the background.

You may notice:

Apps opening slowly
Delayed startup
Lag during simple tasks
Fans running harder than usual
High CPU usage while idle

Malwarebytes identifies significant unexplained slowdowns as a common sign of infection.

However, slow performance alone is not proof of malware.

Possible non-malware causes include:

Low storage
Insufficient RAM
Windows updates
Aging hardware
Too many startup applications
Software bugs

Look for sudden changes combined with other symptoms.

2. Your PC Freezes, Crashes, or Restarts

Malware can interfere with normal operating-system or application behavior.

Warning signs may include:

Repeated freezing
Blue-screen errors
Apps closing unexpectedly
Random restarts
System instability

NCSC guidance specifically lists unexplained reboots and programs closing unexpectedly among signs that may indicate infection.

Again, hardware failures and driver problems can cause the same behavior.

3. You See Unexpected Pop-Ups

Persistent pop-ups can indicate:

Adware
Browser hijacking
Malicious notifications
Scam websites
Unwanted extensions

Be especially cautious with messages such as:

“Your PC has 17 viruses!”

“Call Microsoft Support immediately!”

A browser pop-up claiming infection is not necessarily proof that your computer has malware.

It may itself be a scam designed to convince you to download software or provide personal information.

4. Your Browser Redirects You

Malware or unwanted software can change browser behavior.

Possible signs include:

Search results opening unrelated sites
New tabs appearing automatically
Unexpected advertisements
Redirects to suspicious pages
Search engines changing without permission

ESET lists browser redirects and unexpected homepage changes among common malware-related symptoms.

5. Your Browser Homepage Changes

If your homepage, search engine, or browser settings change without you doing it, investigate.

This may result from:

Browser hijackers
Unwanted applications
Malicious extensions
Altered proxy or DNS settings

A particularly suspicious sign is when you restore the settings but they change again.

6. Unknown Programs Appear

Software that you do not remember installing can be a warning sign.

Possible explanations include:

Bundled software
Potentially unwanted programs
Malware
Software installed by another authorized user

Do not automatically delete every unfamiliar application.

Instead, check:

Publisher
Installation date
Digital signature
File location
Whether it belongs to trusted software

7. CPU, Disk, or Network Usage Is Unusually High

Some malware performs resource-intensive activities such as:

Cryptocurrency mining
Botnet activity
Data exfiltration
File encryption
Downloading additional malware

Malwarebytes specifically identifies unexplained consumption of processing power, memory, storage, or internet bandwidth as a potential sign.

High resource usage can also have legitimate causes, so identify which process is responsible before reaching a conclusion.

8. Antivirus Is Disabled

This is a stronger warning sign.

Some malware attempts to disable:

Antivirus
Endpoint protection
Firewall controls
Security updates

ESET specifically lists security products being unexpectedly disabled or unable to restart as a potential malware symptom.

If security software suddenly stops working without your permission, investigate promptly.

9. You Cannot Open Security Tools

Malware may attempt to block access to tools that could remove it.

Possible symptoms include being unable to open:

Task Manager
Command Prompt
Registry Editor
Security websites
Antivirus applications

ESET lists inability to access these types of tools among potential malware indicators.

10. Files Become Missing or Encrypted

Unexpected file changes can indicate a serious infection.

Watch for:

Documents that no longer open
Strange file extensions
Missing files
Ransom notes
Large numbers of files changing rapidly

Malwarebytes lists encrypted files and being locked out of files or the device as a strong indicator of ransomware.

If ransomware is suspected, avoid continuing ordinary computer use and follow appropriate incident-response procedures.

11. Your Disk Space Suddenly Disappears

Malware may:

Download additional payloads
Create temporary files
Generate logs
Store stolen information
Consume storage through malicious processes

Malwarebytes specifically lists unexplained disk-space loss as one potential symptom.

Of course, backups, updates, caches, and normal applications can also consume large amounts of storage.

12. Strange Browser Extensions or Toolbars Appear

Unexpected extensions can:

Redirect searches
Track browsing
Display advertisements
Modify webpages
Collect information

ESET includes unfamiliar browser toolbars among common warning signs of browser-related malware.

Review your browser’s extension list and remove only items you have confirmed are unwanted.

13. Emails Are Sent Without You

If friends or coworkers receive messages that you did not send, either your computer or your account may be compromised.

NCSC specifically lists unexpected messages sent from your account as a possible infection indicator.

This does not always mean malware is installed locally.

It could also mean your email password has been stolen elsewhere.

Review account logins and reset compromised credentials.

14. You See Suspicious Account Activity

Credential-stealing malware can lead to:

Unknown logins
Password-reset emails
Unexpected MFA prompts
New trusted devices
Changed account settings

This is important because a malware infection can extend beyond the computer itself.

The attacker may use stolen credentials to compromise:

Email
Cloud accounts
Banking
Social media
Business applications

15. Your Computer Acts on Its Own

You may notice:

Programs opening unexpectedly
Settings changing
New processes starting
Network traffic while idle

However, scheduled tasks, software updates, remote-management applications, and normal background services can create similar behavior.

Treat unexplained activity as a reason to investigate rather than automatic proof of malware.

Malware Signs vs. Normal Computer Problems

This comparison is valuable because many users mistake ordinary computer problems for malware.

Symptom	Possible Malware Cause	Possible Normal Cause
Slow PC	Cryptominer, spyware	Low RAM, old hardware
High CPU	Botnet or mining malware	Updates, legitimate software
Crashes	Malware interference	Driver/software fault
Pop-ups	Adware	Browser notifications
Redirects	Browser hijacker	Misconfigured extension
Low disk space	Malware files	Updates, videos, backups
Security disabled	Malware tampering	Software conflict
Files missing	Ransomware/destructive malware	Sync/storage failure

ESET explicitly warns that many common malware symptoms can also be caused by legitimate software or hardware problems.

How Can I Confirm Malware Is on My Computer?

The strongest way to determine whether your computer has malware is to combine symptoms with security evidence.

Step 1: Update Your Security Software

Make sure your antivirus or endpoint-security software is current.

Step 2: Run a Full Malware Scan

NCSC guidance says running an antivirus scan is the easiest way to confirm a suspected infection on a PC or laptop.

Use a full scan rather than relying solely on a quick scan when compromise is suspected.

Step 3: Review Detected Threats

Look for:

Malware detections
Suspicious files
Potentially unwanted applications
Security warnings

Step 4: Investigate Suspicious Programs

Review:

Recently installed applications
Startup software
Browser extensions
Unknown processes

Step 5: Check Account Activity

Review important online accounts for:

Unknown logins
Password changes
MFA requests
Unknown devices

Step 6: Run a Deeper Scan if Needed

If the computer continues behaving suspiciously despite a clean scan, consider:

Offline scanning
A reputable second-opinion scanner
EDR/security investigation in business environments

Kaspersky similarly recommends obtaining a second opinion from another scanner if the first scan finds nothing but suspicion remains.

Quick Scan vs. Full Scan for Malware

Quick Scan	Full Scan
Checks common threat locations	Checks a much broader area
Usually faster	Takes longer
Good for routine checks	Better when infection is suspected
Limited scope	More comprehensive
Lower resource use	Higher resource use

If you’re asking “How do I know if malware is on my computer?”, a full scan is usually the stronger option when suspicious symptoms are already present.

What Should I Do If Malware Is Found?

If your security software confirms malware:

Quarantine the detected threat.
Remove confirmed malware.
Restart the computer if instructed.
Run another full scan.
Install operating-system updates.
Update browsers and applications.
Change exposed passwords from a trusted device.
Enable MFA.
Review important accounts for unauthorized access.
Monitor the device for recurring symptoms.

For company-owned computers, notify IT or security staff and follow established incident-response procedures.

How To Prevent If Malware Is On My Computer?

If your computer has malware, you can identify it by some telltale signs. Once you are aware of such signs, you can take immediate action against it. Here are some of the common malware symptoms,

Sluggish Computer

If you notice a slowdown in computer performance, it may be due to malware running in the background. If your system has malware, you may experience frequent system crashes and freezes as well. In fact, opening various applications and windows is also time-consuming. All these are indicative of the presence of malware in your computer.

Unwanted Changes To Your Browser

Some malware as the tendency to change browser settings especially homepage setting. This is to redirect you to dubious websites containing even more potent malware. If your system gets infected with malware, you may discover changes in some or all of your browser settings. Your browser homepages may have been set to websites that you have never visited. This is a sign of malware infection.

Annoying Pop-ups

If your computer displays popup ads that contain inappropriate content, are difficult to close, and having flashing colors, it is a clear sign of malware infection.

Apart from the signs mentioned above, malware can change the names of your system files. It can also execute applications that will transfer your files from one folder to another. Some malware can even completely wipe your data stored on the computer.

How to Protect Your Computer From Malware?

The best way to remove malware from your computer and to prevent future infection is to install a good antivirus program like Xcitium Antivirus on your computer.

For organizations, Xcitium Advanced Endpoint Protection (AEP) is an ideal solution as it protects every endpoint from malware threats. With built-in containment engine and ‘Default Deny’ platform, Xcitium AEP provides complete protection against any malware threat including zero-day malware.

For more details about Xcitium Advanced Endpoint Protection, contact us at +1 888-256-2608.

Frequently Asked Questions

How do I know if malware is on my computer?

Common signs include sudden slowdowns, crashes, pop-ups, browser redirects, unknown software, security tools being disabled, unusual resource usage, missing or encrypted files, and suspicious account activity. The simplest way to confirm a suspected infection is to run an updated antivirus scan.

What is the most obvious sign of malware?

There is no universal single sign. Stronger indicators include antivirus detections, ransomware encrypting files, security software being disabled unexpectedly, or applications/settings changing without authorization.

Does a slow computer mean I have malware?

No. Malware can cause slow performance, but low storage, insufficient RAM, software updates, hardware issues, and legitimate applications can do the same.

Can malware be present without symptoms?

Yes. Some malware is designed to remain hidden and may not noticeably affect performance.

What should I do if I think I have malware?

Update your antivirus software, run a full scan, quarantine confirmed threats, restart and rescan where appropriate, update the system, and change potentially exposed credentials from a trusted device.

What if antivirus finds nothing?

Run a full scan, update the security software, consider an alternative trusted scanner or offline scan, and investigate non-malware causes if symptoms continue.

Can malware disable antivirus?

Yes. Some malware attempts to disable security software to avoid detection. An antivirus product that unexpectedly stops working can therefore be an important warning sign.

Can malware steal passwords?

Yes. Spyware, keyloggers, infostealers, and Trojans can steal passwords and other credentials.

Can malware cause browser redirects?

Yes. Browser hijackers, adware, malicious extensions, and other unwanted programs can redirect web traffic.

How do I know malware is completely removed?

Run another full scan after remediation, confirm that security software is functioning normally, and monitor the computer and important accounts for recurring suspicious activity.