Signs Your Computer Might Be Infected
A virus on your computer usually announces itself through changes you will notice before anything else breaks. Your machine may run slower than normal, programs may freeze or crash without reason, or your browser may redirect you to websites you did not click on. You might see pop-up windows that appear even when you are not online, or your antivirus software (if you have it) may suddenly stop working or refuse to open.
Other signs include unexpected activity: your hard drive working constantly even when you are not using programs, your fan running at high speed for no reason, or your mouse cursor moving on its own. Some viruses change your desktop background, add toolbars to your browser, or alter your home page without your action. If your computer restarts on its own, or if you see error messages you have never seen before, those are also red flags worth investigating.
Not every slowdown or crash means you have a virus — a full hard drive, outdated software, or a failing hard drive can cause the same symptoms. But if multiple signs appear at once, or if the behavior is new and sudden, a virus is a reasonable suspect.
Key Takeaways
- Slow performance, unexpected pop-ups, browser redirects, and programs that freeze or crash are common signs of a virus infection.
- Run a full scan with your antivirus software or Windows Defender (built into Windows) to check for threats and get a clear report.
- If your antivirus software stops working or will not open, the virus may be blocking it — restart your computer in Safe Mode and scan from there.
- Malware removal tools like Malwarebytes can find threats that standard antivirus software misses, especially spyware and adware.
- If a scan finds nothing but problems persist, the issue may be hardware failure, a corrupted Windows installation, or a rootkit that requires professional help.
How to Run a Virus Scan on Windows
Windows comes with built-in antivirus software called Windows Defender (or Microsoft Defender on newer versions). Open it by typing "Windows Defender" into your search box and clicking the app. Click "Virus & threat protection" on the left side, then click "Scan options." Choose "Full scan" — this checks every file on your computer and takes 30 minutes to several hours depending on how much data you have.
Click "Scan now" and let it run. Do not interrupt it or restart your computer. When the scan finishes, Windows Defender will show you a report. If it found threats, it will tell you what they are and whether it removed them. If it removed them automatically, restart your computer. If it could not remove them, you may need to use a specialized removal tool or restart in Safe Mode and scan again.
If you have third-party antivirus software installed (Norton, McAfee, Kaspersky, Bitdefender, or others), use that instead of Windows Defender — do not run both at the same time, as they can interfere with each other. Open your antivirus program and look for a "Scan" or "Full Scan" button. The process is similar: choose full scan, let it run, and review the report when it finishes.
What to Do If Your Antivirus Software Will Not Open
Some viruses deliberately block antivirus programs from running. If you cannot open Windows Defender, your antivirus software, or both, restart your computer in Safe Mode. Safe Mode loads only the essential Windows files and drivers, which often allows antivirus software to run even when a virus is present.
To restart in Safe Mode on Windows 10 or 11, hold the Shift key and click the power button in the bottom right corner of your screen. Click "Restart." Your computer will show a blue screen with options. Click "Troubleshoot," then "Advanced options," then "Startup Settings," then "Restart." When your computer restarts, press the number 4 or F4 to enter Safe Mode with Networking (Networking lets you read tools if you need them).
Once in Safe Mode, open Windows Defender or your antivirus software and run a full scan. Safe Mode is a limited environment, so the scan may run slower, but the antivirus software usually works when it would not in normal mode. If the scan finds and removes threats, restart your computer normally and run another scan to confirm the infection is gone.
Using Malwarebytes to Find Threats Your Antivirus Missed
Malwarebytes is a specialized tool that finds spyware, adware, and other malicious software that standard antivirus programs sometimes miss. It is free to read and use for a single scan. Go to malwarebytes.com, click "Free read," and install the program. Open it and click "Scan" — Malwarebytes will check your computer and show you what it found.
The free version scans on demand but does not protect your computer in real time. If Malwarebytes finds threats, it will ask you to pay for the full version to remove them. You can also use the free version of Malwarebytes alongside your regular antivirus software without conflict — they are designed to work together.
Run Malwarebytes after your antivirus scan finishes. If both scans find nothing but your computer is still slow or behaving strangely, the problem is likely not a virus. Move on to checking your hard drive space, updating your software, or considering hardware failure.
When to Suspect a Rootkit or Advanced Infection
A rootkit is a type of malware that hides itself from antivirus software by running at the deepest level of your operating system. Standard scans often cannot find it. If your computer shows all the signs of infection but every scan comes back clean, a rootkit may be present.
Some antivirus programs include rootkit detection — check your software's settings or help section to see if you can enable a rootkit scan. Kaspersky, Bitdefender, and some versions of Norton include this feature. If your current antivirus does not offer it, you may need to read a specialized rootkit scanner like GMER or Avast's rootkit scanner (both free).
If you suspect a rootkit and your antivirus cannot find it, backing up your important files and reinstalling Windows is often the most reliable solution. This is a larger task, so consider whether the computer is worth the effort or whether replacement makes more sense.
Checking Your Browser and Startup Programs
Some viruses do not show up in antivirus scans because they are disguised as browser extensions or startup programs. Open your browser (Chrome, Firefox, Edge, or Safari) and look for unfamiliar toolbars, extensions, or changes to your home page or search engine.
In Chrome, click the three-line menu in the top right, go to "Settings," then "Extensions." Look for anything you do not recognize and click the trash icon to remove it. Check your home page by going to "Settings" and looking at the "On startup" section — it should show a page you chose, not a random search engine.
In Windows, you can also check what programs start when your computer boots. Type "Task Manager" into your search box and open it. Click the "Startup" tab. Look for programs you do not recognize or did not install yourself. Right-click them and select "Disable." This will not remove them, but it will stop them from running automatically. Restart your computer to see if performance improves.
When to Seek Professional Help
If you have run a full antivirus scan, used Malwarebytes, checked Safe Mode, and your computer is still slow or behaving strangely, the problem may not be a virus. It could be a failing hard drive, corrupted Windows files, or hardware that needs replacement. At this point, taking your computer to a repair shop is worth the cost — a technician can run deeper diagnostics and tell you whether the machine can be fixed or whether replacement is the better choice.
You should also seek professional help if you are not comfortable restarting in Safe Mode or downloading and running scanning tools. A technician can do this work for you and explain what they found. The cost is usually between fifty and two hundred dollars depending on your location and the shop, which is reasonable insurance against accidentally making the problem worse.
Frequently Asked Questions
Can a virus spread to my phone or tablet from my computer?
Most computer viruses cannot run on phones or tablets because they use different operating systems. However, some malware targets multiple platforms, and you can accidentally read malicious apps to your phone if you visit an infected website on it. Keep your phone and tablet updated and read apps only from the official App Store or Google Play Store.
Is it safe to use my computer while a virus scan is running?
You can use your computer during a scan, but it will slow down both the scan and your work. The scan needs to read every file on your hard drive, which competes with whatever else you are doing. It is better to start a scan and let it run without interruption, or run it overnight when you are not using the computer.
What if the antivirus scan says it removed the virus but my computer is still slow?
The virus may have damaged files or settings that the antivirus could not repair. Run the scan again to confirm nothing is still there. If the second scan is clean but slowness persists, check how much hard drive space you have free (right-click your C: drive and click "Properties" — you want at least 10 percent free), restart your computer, and update Windows and your software. If none of that helps, the slowness may be from hardware failure rather than infection.
Do I need to pay for antivirus software if Windows Defender is free?
Windows Defender is adequate for most users and costs nothing. Paid antivirus software offers more features like real-time protection, browser extensions, and customer support, but Windows Defender provides real-time protection too. If you read files frequently or visit risky websites, paid software may be worth it, but it is not required.
Can I get a virus just by visiting a website?
You can get infected by visiting a compromised website, but only if your browser or operating system has an unpatched security flaw. Keeping Windows and your browser updated closes most of these holes. You are at higher risk if you click on suspicious links or read files from untrusted sources. Avoid clicking links in emails from people you do not know, and be cautious with file downloads.