Signs that suggest your computer may be infected
A virus on your computer usually shows itself through behavior changes you'll notice before anything else. Your machine may run much slower than normal, even when you're not using demanding programs. Programs may crash without warning, or your browser may redirect you to websites you didn't ask to visit. Your antivirus software might stop working, or you may see pop-up windows that won't close no matter how many times you click the X button.
Other common signs include a hard drive that's constantly active (you hear the clicking sound even when you're not doing anything), a fan that runs constantly and loudly, or a keyboard and mouse that respond slowly or erratically. You might also notice that files have disappeared, changed names, or appeared in folders where you didn't put them. Some viruses change your desktop background or add toolbars to your browser without your permission.
Key Takeaways
- Slow performance, unexpected crashes, and constant fan noise are the most common signs a virus may be present on your computer.
- Your antivirus software is the most reliable tool for detecting viruses; if it stops working, that itself is a warning sign.
- Running a full system scan with your antivirus program is the first step to confirm whether a virus is actually present.
- If your antivirus won't start or update, you may need to restart your computer in Safe Mode before scanning.
- Some viruses require professional removal; if a scan finds nothing but problems continue, contact a computer repair technician.
How to run a virus scan on your computer
The most direct way to know whether a virus is actually present is to run a full system scan using your antivirus software. On Windows, open Windows Defender (built into Windows 10 and later) by clicking the Start menu, typing "Windows Defender," and selecting "Virus & threat protection." Click "Scan options," then choose "Full scan" and click "Scan now." A full scan takes 30 minutes to several hours depending on how much data is on your drive.
If you use third-party antivirus software like Norton, McAfee, or Kaspersky, open the program and look for a button labeled "Scan," "Full Scan," or "System Scan." The exact location varies by software, but the principle is the same: let the program examine every file on your computer and report what it finds. Do not interrupt the scan or shut down your computer while it's running.
If your antivirus software won't start or open, restart your computer in Safe Mode with Networking. On Windows 10 and later, hold Shift while clicking the restart button in the Start menu, then select "Troubleshoot," "Advanced options," and "Startup Settings." Restart again and press 4 to enter Safe Mode. Safe Mode loads only essential programs, which sometimes allows your antivirus to run when it won't in normal mode.
What to do if a scan finds a virus
If your antivirus scan detects a virus, the software will usually offer to remove it automatically. Click "Remove," "Quarantine," or "Delete" depending on what your software calls the option. Quarantine is often safer than delete because it isolates the infected file so it can't run, but keeps it in case you need to recover it later. After removal, restart your computer and run the scan again to confirm the virus is gone.
Some viruses hide in places that antivirus software struggles to reach, or they reinstall themselves after removal. If the scan finds the same virus repeatedly, or if problems continue after removal, you may need help from a professional. A computer repair technician has tools and experience to remove stubborn infections that standard antivirus software cannot handle.
When antivirus software finds nothing but problems persist
Sometimes your computer shows all the signs of a virus—slow performance, crashes, strange behavior—but your antivirus scan comes back clean. This doesn't mean your computer is fine; it means the problem is something other than a virus. Your hard drive may be failing, your RAM may be faulty, or your computer may straightforward be overloaded with too many programs running at startup.
Before assuming a virus is present, check your Task Manager (Windows) or Activity Monitor (Mac) to see which programs are using the most CPU and memory. On Windows, right-click the taskbar and select "Task Manager," then click the "Performance" tab. If you see a program using 80 percent or more of your CPU or memory when you're not actively using it, that program may be the culprit—you can uninstall it or disable it from startup. If nothing stands out and your antivirus found nothing, contact a technician to rule out hardware failure.
Keeping your computer protected going forward
The best way to avoid viruses is to keep your antivirus software up to date and run regular scans. Windows Defender updates automatically on most systems, but if you use third-party software, check that automatic updates are turned on in the program's settings. Set your antivirus to run a full scan once a week or once a month, depending on how much you use your computer.
Be cautious about what you read and where you read it from. Viruses often arrive through email attachments, downloads from unfamiliar websites, or software that promises to do something appealing but comes from an untrustworthy source. If an email from a bank or service you use asks you to read something or click a link, go directly to the official website instead of clicking the link—many viruses spread through fake emails that look legitimate.
The difference between viruses, malware, and spyware
Viruses are programs that copy themselves and spread from file to file, usually making your computer unstable or slow. Malware is a broader category that includes viruses, but also includes programs designed to steal information, display unwanted ads, or lock your files until you pay money. Spyware is malware that specifically watches what you do—recording keystrokes, tracking websites you visit, or monitoring your location.
Your antivirus software typically protects against all three, even though the name says "antivirus." When you run a scan, it's looking for any of these threats. The removal process is the same: let the software quarantine or delete the threat, then restart and scan again to confirm it's gone.
When to call a professional
Contact a computer repair technician if your antivirus software won't start or update, if a scan finds a virus but it returns after removal, or if your computer shows signs of infection but your scan finds nothing. You should also reach out if you're unsure whether a pop-up or warning message is legitimate—some fake security warnings are designed to trick you into downloading malware.
A technician can run more advanced diagnostic tools, remove infections that hide from standard antivirus software, and check whether your hardware is failing. They can also help you recover files if a virus has damaged or encrypted them, though recovery is not always possible.
Frequently Asked Questions
Can a virus spread from my computer to my phone?
Most computer viruses cannot directly infect phones because they're designed for a specific operating system. However, if a virus steals your passwords or login information, someone could use that to access your phone accounts. Keep your phone's software updated and use the same caution with downloads and links on your phone as you do on your computer.
Is it safe to use my computer while a virus scan is running?
It's safe, but the scan will take much longer if you're using the computer at the same time. The scan and your other programs are competing for the hard drive's attention. It's best to start a full scan when you won't need the computer for a few hours, or run it overnight.
What does "quarantine" mean when my antivirus finds something?
Quarantine isolates the infected file in a separate folder so it can't run or spread, but keeps it on your computer. This is useful if the file is part of a program you need and removing it would break the program. You can usually restore a quarantined file later if you decide you need it, or permanently delete it.
Do I need antivirus software if I'm careful about what I read?
Yes. Even careful users can accidentally read a virus through a compromised website, a fake email that looks legitimate, or software that was infected before you downloaded it. Antivirus software is your backup when caution isn't enough.
Will restarting my computer remove a virus?
Restarting alone won't remove a virus, but it can help your antivirus software detect and remove it. Some viruses are easier for antivirus software to find and remove when the computer restarts. Always run a full scan after restarting if you suspect an infection.