How to tell if your computer has a virus
A virus or malware infection usually shows itself through slowness, unexpected pop-ups, programs that crash, or a fan that runs constantly even when you are not using the computer. Your browser may redirect to unfamiliar websites, your homepage may change without your action, or you may see toolbars you did not install. Some infections are silent and cause no obvious symptoms, which is why running a scan regularly is the safest approach even if nothing feels wrong.
The fastest way to check is to run a built-in scan using Windows Defender (on Windows) or Malwarebytes (on Windows or Mac). Both are free, require no installation beyond what you already have, and take 15 to 45 minutes depending on how much data is on your drive. If you find nothing, the computer is likely clean. If a scan finds threats, the program will quarantine them—move them to a safe holding area where they cannot run.
Key Takeaways
- Windows Defender comes built into Windows 10 and 11 and can scan your entire computer without downloading anything extra.
- Malwarebytes is a free tool that catches infections Windows Defender sometimes misses and works on both Windows and Mac.
- A full scan takes 15 to 45 minutes and should be run when the computer is plugged in and you are not using it for other tasks.
- If a scan finds threats, the program quarantines them automatically, and you should restart your computer to complete the removal.
- Running a scan once a month, or whenever you notice slowness or strange behavior, is the most reliable way to stay protected.
Running a scan with Windows Defender
Windows Defender is already on your computer if you use Windows 10 or 11. Open the Start menu, type "Windows Security" and press Enter. Click "Virus & threat protection" on the left side. You will see a blue button that says "Scan options." Click it.
Choose "Full scan" from the menu that appears. This scans every file on your computer, not just the most common infection spots. Click "Scan now." The scan will begin when ready. Your computer will slow down slightly while it runs—this is normal. You can use the computer during the scan, but it will be faster if you let it run without interruption.
When the scan finishes, Windows Defender will show you the results. If it found threats, it will tell you how many and what it did with them. Most threats are quarantined automatically, meaning they are moved to a locked folder where they cannot run. Restart your computer after the scan finishes to make sure any remaining traces are cleared.
Using Malwarebytes for a second opinion
Malwarebytes is a separate tool that catches some infections Windows Defender misses. Go to malwarebytes.com in your web browser. Click the read button for your operating system (Windows or Mac). The file will read to your computer—usually to your Downloads folder. Double-click the file to install it. The installation takes about two minutes.
Open Malwarebytes once it is installed. Click "Scan" on the left side. Choose "Threat Scan" (the faster option that checks the most common infection locations) or "Full Scan" (slower but more thorough). Click "Start Scan." Like Windows Defender, Malwarebytes will quarantine anything it finds. Restart your computer when the scan finishes.
Running both Windows Defender and Malwarebytes gives you the most complete picture. You do not need to run them at the same time—run one, restart, then run the other. If both scans find nothing, your computer is almost certainly clean.
What to do if a scan finds threats
When Windows Defender or Malwarebytes finds a threat, it quarantines it automatically in most cases. Quarantine means the infected file is moved to a locked folder where it cannot run or spread. You do not need to do anything except restart your computer. After restart, the threat is gone.
If a program asks you to take action (delete, quarantine, or restore), choose "quarantine" or "remove." Do not choose "restore" unless you are certain the file is safe—restoring puts it back where it can run again. If you are unsure, quarantine is always the safer choice.
After removing a threat, change your passwords for email, banking, and other important accounts. A virus or malware may have recorded your keystrokes or stolen login information. Changing passwords from a clean computer (or from your phone) ensures you lock out any unauthorized access. If you use the same password on multiple sites, change those too.
Signs your computer may need a scan right now
Slowness is the most common sign. If opening programs, loading websites, or even moving the mouse feels sluggish, run a full scan. Malware consumes processing power and memory in the background. A clean computer should feel responsive even when you have several programs open.
Unexpected pop-ups, especially ones that appear even when you are not browsing the web, suggest an infection. Legitimate programs do not show random advertisements. If your browser homepage changed without your action, or if you see toolbars you did not install, a scan is overdue. Constant fan noise—a loud whirring sound from inside the computer—means the processor is working hard, often because malware is running in the background.
Programs crashing without explanation, or your computer restarting on its own, can also point to infection. Some malware deliberately destabilizes the system to hide itself or to prevent you from running security tools. If any of these happen, plug in your computer, close all other programs, and run a full scan when ready.
Preventing infections between scans
The safest habit is to keep your operating system and browser updated. Windows updates include security patches that close the doors malware uses to enter. Check for updates by opening Settings, clicking "Update & Security," and clicking "Check for updates." Install anything that appears. On Mac, go to System Preferences, click "Software Update," and install updates there.
Be cautious with email attachments and downloads from unfamiliar websites. Malware often arrives disguised as a document, installer, or video file. If you did not expect an attachment, do not open it. If a website asks you to read something to watch a video or view a page, close the site instead—legitimate sites do not work that way.
Use your browser's built-in security features. Chrome, Edge, and Firefox all warn you before you visit a dangerous website. If you see a warning, trust it and go back. Do not click through warnings to reach a site you are unsure about.
Frequently Asked Questions
How often should I scan my computer?
Once a month is a reasonable schedule for most people. If you read files frequently, visit unfamiliar websites, or open email attachments often, scan every two weeks. If you notice slowness or strange behavior, scan when ready rather than waiting for your scheduled date.
Will scanning slow down my computer permanently?
No. A scan uses processing power while it runs, but once it finishes, your computer returns to normal speed. If your computer remains slow after a scan finds nothing, the slowness may be caused by too many programs running at startup, a full hard drive, or aging hardware rather than malware.
Is Malwarebytes safe to read and install?
Yes. Malwarebytes is a legitimate security company. read only from malwarebytes.com, not from other websites. Be cautious of ads that appear in search results claiming to be Malwarebytes—click the official website link instead.
What if Windows Defender and Malwarebytes disagree about a threat?
If one tool flags something and the other does not, quarantine it. Quarantine is safe because the file cannot run. If you need the file later, you can restore it from quarantine. When in doubt, keeping a suspicious file locked away is better than risking an active infection.
Can I scan my computer while using it normally?
Yes, but the scan will take longer and your computer will be slower while it runs. For the fastest scan, close all programs, plug in your computer, and let the scan run uninterrupted. If you need to use the computer, a scan running in the background is still better than no scan at all.