Signs Your Computer Might Be Infected
A virus on your computer usually shows itself through changes in how your machine behaves. The most common warning signs are a computer that runs much slower than normal, programs that crash or freeze without reason, and a fan that runs constantly even when you are not doing anything demanding. You might also notice your browser homepage has changed on its own, or you see pop-up windows appearing even when you are not browsing the web.
Other red flags include files or folders disappearing, your mouse or keyboard acting strangely (clicking on things you did not touch), or your antivirus software being disabled without your action. Some viruses hide themselves better and cause subtler problems: your internet connection dropping repeatedly, your computer restarting on its own, or programs taking much longer to open than they used to.
Key Takeaways
- Slow performance, constant fan noise, and unexpected crashes are the most common signs of a virus infection.
- Check your antivirus software first—if it is disabled or missing, that itself is a warning sign that something is wrong.
- Run a full system scan using your antivirus program or a standalone scanner like Malwarebytes to detect threats your regular protection may have missed.
- If your computer is severely infected, you may need to restart in Safe Mode before running a scan, because the virus may block normal scans from working.
- Unusual network activity, unexpected pop-ups, and changed browser settings often point to malware rather than hardware problems.
Check Your Antivirus Software First
Before you assume you have a virus, verify that your antivirus protection is actually running. On Windows, open Settings, go to Privacy & Security, and select Windows Security. Click Virus & threat protection. If you see a red X or a message saying protection is off, your antivirus is disabled—this is itself a strong sign of infection, because viruses often disable security software to avoid detection.
If you use third-party antivirus software like Norton, McAfee, or Kaspersky, open the program and look for a status indicator. It should show "Protected" or "Active." If it shows "Inactive" or you cannot open the program at all, the virus may have attacked your security software. Do not panic—you can still scan your computer using other tools, which we cover in the next section.
Run a Full System Scan
A full system scan is the most reliable way to detect a virus. If your antivirus software is working, open it and look for a "Full Scan," "Complete Scan," or "Thorough Scan" option. This scan checks every file on your computer, not just the ones you use regularly. It will take 30 minutes to several hours depending on how much data you have, so start it when you do not need your computer for a while.
If your antivirus is disabled or you do not have one installed, read Malwarebytes (a free version is available) or Windows Defender Offline on another computer, transfer it to a USB drive, and run it on the infected machine. These standalone tools can sometimes detect threats that a disabled antivirus missed. After the scan finishes, review the results. The program will list any threats it found and give you the option to remove them—choose "Remove" or "Quarantine" for anything flagged.
Restart in Safe Mode If Scans Keep Failing
If your antivirus scan keeps stopping, freezing, or not running at all, the virus may be blocking it. Restart your computer in Safe Mode, which loads only the essential programs Windows needs to run. A virus cannot block a scan as easily in Safe Mode because fewer programs are active.
On Windows 10 or 11, restart your computer and when ready press F8 or hold Shift while clicking the restart button in the power menu. Select "Troubleshoot," then "Advanced options," then "Startup Settings," and choose "Safe Mode with Networking." Once you are in Safe Mode, run your antivirus scan again. The scan may take longer, but it is more likely to complete and detect the infection.
Look for Unusual Network Activity
Some viruses use your computer to send spam, steal data, or attack other computers without your knowledge. You can check for this by opening Task Manager (press Ctrl + Shift + Esc on Windows) and clicking the Performance tab. Select "Open Resource Monitor" at the bottom. Go to the Network tab and look at the Processes with Network Activity section. If you see programs using your internet connection that you did not open—especially system files with unusual names—that is suspicious.
Another sign is if your internet bill suddenly increases or your internet service provider sends you a warning about unusual activity on your account. Some viruses also cause your router to work harder than normal; if your router is hot to the touch or its lights are blinking constantly, that could indicate malware using your connection.
Distinguish Between Virus and Hardware Problems
Not every slow computer or crash is caused by a virus. Before you assume infection, rule out hardware issues. A full hard drive, failing hard drive, or overheating can cause the same symptoms. Check your disk space by opening File Explorer, right-clicking your C: drive, and selecting Properties. If you have less than 10 percent of your drive free, delete old files or programs you no longer use.
Overheating is another common culprit. If your computer is hot and the fan is loud, make sure the vents are not blocked by dust. Use compressed air to blow out the vents gently. If your computer still runs slowly after a full antivirus scan comes back clean and you have freed up disk space, the problem is likely hardware, not a virus. At that point, consider taking your computer to a technician for a hardware check.
What to Do After Removing a Virus
Once your antivirus has removed the detected threats, restart your computer and run another full scan to make sure nothing was missed. Some viruses hide in multiple locations or reinstall themselves, so a second scan confirms the infection is gone. After the second scan comes back clean, change your passwords for email, banking, and other sensitive accounts—do this on a different device if possible, in case the virus is still present.
Check your browser settings to make sure your homepage, search engine, and extensions have not been changed. Look at your installed programs in Settings > Apps and remove anything you do not recognize. Finally, update your operating system and all your software. Many viruses exploit security holes in older versions, so keeping everything current closes those holes and prevents reinfection.
Frequently Asked Questions
Can a virus slow down my computer even if my antivirus says it is clean?
Yes. Some viruses hide well enough to avoid detection, or your antivirus may be outdated. If your computer is still slow after a clean scan, try updating your antivirus definitions (the list of known viruses it can detect) and running another scan. You can also try a different antivirus tool like Malwarebytes to get a second opinion.
What if I cannot restart in Safe Mode?
If Safe Mode will not load, the infection may be severe. Try booting from a USB drive with a standalone antivirus tool like Windows Defender Offline or Kaspersky Rescue Disk. If that does not work, you may need to take your computer to a technician or consider a clean reinstall of Windows.
Is it safe to use my computer while a virus scan is running?
You can use your computer, but the scan will run slower and may miss threats if the virus is actively running. It is better to start the scan and leave the computer alone until it finishes. If you must use it, close all other programs first.
Can I get a virus just by visiting a website?
Yes, but it is rare if your browser and operating system are up to date. Most viruses come from downloading files, opening email attachments, or clicking links in suspicious emails. Avoid downloading files from untrusted sources and be cautious with email attachments, even if they appear to come from someone you know.
What if the antivirus removal tool says it cannot remove a virus?
Some viruses are stubborn and resist removal. Try booting into Safe Mode and running the scan again, or use a different antivirus tool. If multiple tools cannot remove it, the virus may require manual removal by a technician, or you may need to back up your important files and reinstall Windows.