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 sudden slowdown, programs that crash or freeze without reason, unexpected pop-up windows that appear even when you are not browsing, and a fan that runs constantly even when you are not doing anything demanding. Your computer may also restart on its own, display error messages you have never seen before, or take much longer than usual to start up.
Other red flags include programs that launch without you opening them, your internet connection dropping repeatedly, or your antivirus software being disabled without your action. If your browser homepage has changed on its own, or you see toolbars you did not install, those are also common signs of infection. Pay attention to your keyboard and mouse too — if they respond slowly or seem to lag behind your input, that can point to a virus consuming your system resources.
Key Takeaways
- Slowness, unexpected crashes, constant fan noise, and unwanted pop-ups are the most reliable signs of a virus infection.
- Run a full scan with your antivirus software or Windows Defender (built into Windows) to check for threats — this takes 30 minutes to several hours depending on your drive size.
- If your antivirus is disabled or will not open, restart your computer in Safe Mode with Networking before scanning.
- Disconnect from the internet when ready if you see signs of infection, then scan before going back online.
- If a scan finds nothing but your computer still behaves strangely, the problem may be hardware failure, too many startup programs, or a corrupted Windows installation rather than a virus.
How to Run a Full Antivirus Scan
The most direct way to check for a virus is to run a complete scan with antivirus software. If you use Windows 10 or Windows 11, you already have Windows Defender built in. Open the Start menu, type "Windows Security," and click the result. Select "Virus & threat protection" on the left side, then click "Scan options." Choose "Full scan" and click "Scan now." This will examine every file on your computer and take between 30 minutes and several hours depending on how much data you have.
If you have third-party antivirus software like Norton, McAfee, Avast, or Bitdefender, open that program instead and look for a "Full Scan" or "Complete Scan" button — the exact wording varies by software. Start the scan and let it run to completion without interrupting it. Do not use your computer for other tasks while the scan is running, as this slows it down. When the scan finishes, the software will show you a report of any threats found and give you options to remove them.
What to Do If Your Antivirus Will Not Open
Some viruses disable your antivirus software to avoid detection. If you cannot open Windows Defender or your antivirus program, restart your computer in Safe Mode with Networking. This boots Windows with only the essential programs and drivers, which often allows antivirus software to work even if a virus is blocking it in normal mode.
To enter Safe Mode on Windows 10 or 11, restart your computer and press the F8 key repeatedly as it starts up, before the Windows logo appears. (On some computers, you may need to press F2, Delete, or another key — your manufacturer's startup screen will tell you.) Look for an option that says "Safe Mode with Networking" and select it. Once you are in Safe Mode, open Windows Defender or your antivirus software and run a full scan. The scan may take longer in Safe Mode, but it is more likely to detect and remove threats that hide from normal scans.
Checking Your Startup Programs and Browser Extensions
Viruses and unwanted software often add themselves to your startup programs so they run every time you turn on your computer. Open the Task Manager by pressing Ctrl+Shift+Esc, then click the "Startup" tab. Look through the list for programs you do not recognize or did not install yourself. Right-click any suspicious entry and select "Disable." This prevents it from running at startup without removing it, so you can turn it back on later if it turns out to be legitimate.
Browser hijackers — viruses that change your search engine or homepage — hide in your browser extensions. Open your browser settings and look for the Extensions or Add-ons section. Remove anything you do not remember installing or anything with a name that looks suspicious or generic. If your homepage or search engine has changed, reset it back to your preferred option in the browser settings.
When to Disconnect From the Internet
If you see clear signs of infection — such as your antivirus being disabled, constant pop-ups, or your files being locked with a ransom message — disconnect from the internet when ready. Unplug your ethernet cable or turn off your Wi-Fi. Some viruses spread to other computers on your network or send your data to attackers, so disconnecting stops that from happening while you work on removing the threat.
Once you are offline, run a full antivirus scan in Safe Mode. If the scan finds and removes threats, you can reconnect to the internet. If the scan finds nothing but your computer still behaves strangely, the problem may not be a virus — it could be a hardware failure, a corrupted Windows installation, or straightforward too many programs running at startup.
When a Scan Finds Nothing but Problems Continue
Sometimes your computer shows all the signs of a virus — slowness, crashes, strange behavior — but your antivirus scan comes back clean. This does not mean you are virus-free; it means the threat either is not there or your antivirus did not detect it. Before assuming you have an undetectable virus, check a few other common causes.
Open Task Manager (Ctrl+Shift+Esc) and click the "Performance" tab to see how much of your CPU, memory, and disk space is in use. If your disk is at 100% or your memory is nearly full, your computer will feel slow and unresponsive even without a virus. Check the "Processes" tab to see which programs are using the most resources — you may find a program running in the background that you did not know about. If your hard drive is almost full, delete old files or programs you no longer use. If none of that helps, consider running a scan with a second antivirus tool, since different software sometimes catches different threats.
Frequently Asked Questions
Can I get a virus just by visiting a website?
Yes, but it is rare on modern browsers and operating systems. Most websites cannot infect you just by visiting them. The risk is higher if you click on suspicious links, read files from untrusted sources, or have outdated software with known security holes. Keep your browser and operating system updated, and avoid clicking links in emails or messages from people you do not know.
Is Windows Defender enough, or do I need paid antivirus software?
Windows Defender is sufficient for most users and catches the majority of common threats. Paid antivirus software like Norton or Bitdefender may offer extra features like a VPN or password manager, but they are not necessary for basic protection. The most important thing is that you have some antivirus software running and that you keep it updated.
What does it mean if my antivirus says "potentially unwanted program"?
A potentially unwanted program (PUP) is software that is not necessarily malicious but behaves in ways many people find annoying — like showing ads, changing your browser settings, or slowing your computer. You can remove it, but it is not as dangerous as a virus or ransomware. If you installed it yourself and want to keep it, you can ignore the warning.
How often should I scan my computer for viruses?
If you have antivirus software running and updated, it scans files automatically as you read and open them. You do not need to run a full manual scan every week. Run a full scan once a month, or when ready if you notice signs of infection or after you read something from an unfamiliar source.
Can I remove a virus myself, or do I need to take my computer to a repair shop?
Most viruses can be removed by running a full antivirus scan and letting the software quarantine or delete the threats it finds. If a scan in Safe Mode does not work, or if your computer is still broken after removal, then a repair shop can help. But start with the scan yourself — it is free and works in the majority of cases.