Run a scan with Windows Defender or your Mac's built-in protection

Windows and Mac both come with virus scanning tools already installed. You do not need to buy or read anything to check your computer. On Windows 10 and 11, the tool is called Windows Defender (or Microsoft Defender on newer versions). On Mac, the system runs background protection automatically, but you can also use Malwarebytes or check Activity Monitor for suspicious processes.

The fastest way to start is to open your system's built-in scanner and let it run a full scan. This takes 30 minutes to several hours depending on how many files you have, but you can use your computer while it works. A full scan checks every file on your hard drive, not just the programs you use often.

Key Takeaways

  • Windows Defender and Mac's built-in protection are free and already on your computer — open Settings or System Preferences to start a scan.
  • A full scan takes 30 minutes to several hours but checks every file; a quick scan takes 5 to 10 minutes and checks only high-risk areas.
  • If a virus is found, the scanner will quarantine it (move it to a safe folder where it cannot run) and ask what you want to do next.
  • Run a scan at least once a month, or when ready after you notice your computer running slowly, crashing, or showing pop-up ads.
  • If your computer will not start or the scanner will not open, restart in Safe Mode with Networking before trying again.

How to open Windows Defender and start a scan

On Windows 10 or 11, click the Start button (the Windows logo in the bottom left), then type "Windows Defender" or "Virus and threat protection" and press Enter. The Virus and threat protection window will open. Click Scan options near the top.

You will see four choices: Quick scan, Full scan, Custom scan, and Windows Defender Offline scan. Choose Full scan for a complete check of your entire hard drive. Click the blue Scan now button. The scan will begin when ready and show a progress bar. You can close the window and use your computer while it runs — the scan continues in the background.

When the scan finishes, Windows Defender will show you the results. If no threats were found, you will see a green checkmark and the message "No threats found." If anything suspicious was detected, the window will list it and ask whether you want to quarantine it (the default choice) or remove it. Click Quarantine to move the threat to a safe folder where it cannot harm your computer.

How to scan on a Mac

Mac does not have a built-in scanner you open manually the way Windows does. Instead, macOS runs protection in the background automatically. However, you can read Malwarebytes for Mac (free version available) to run a manual scan if you suspect a problem.

Go to malwarebytes.com, click the read button, and follow the installer. Once installed, open Malwarebytes and click Scan. The scan takes 10 to 20 minutes. If threats are found, Malwarebytes will quarantine them automatically. You can also check Activity Monitor (Applications > Utilities > Activity Monitor) to look for programs using unusual amounts of CPU or memory — unfamiliar names running high percentages are sometimes a sign of malware.

Understanding quick scan versus full scan

A quick scan checks only the areas where viruses hide most often: your startup files, temporary folders, and the registry (on Windows). It takes 5 to 10 minutes and catches most common threats. Use a quick scan if your computer is running slowly right now and you want a fast answer.

A full scan checks every file on your hard drive, including files you rarely use. It takes 30 minutes to several hours depending on how much storage you have. Run a full scan once a month as routine maintenance, or if a quick scan found something and you want to make sure nothing else is hiding.

A custom scan lets you choose specific folders to check — useful if you suspect a virus is only in your Downloads folder or on an external drive. A Windows Defender Offline scan restarts your computer and scans before Windows fully loads, which can catch threats that hide from the normal scanner.

What to do if a threat is found

When the scanner finds something, it will show the threat name, the file location, and a recommended action. Most of the time, the recommendation is Quarantine, which moves the file to an isolated folder where it cannot run or spread. This is the safest choice because it keeps the file in case you need it later, but prevents it from causing damage.

If the scanner recommends Remove, click that button. Remove deletes the file entirely. Use Remove only if you are certain the file is malicious — for example, if the scanner found a known virus in a temporary folder you do not need.

After quarantine or removal, restart your computer. Some viruses leave behind pieces that only disappear after a restart. Run another quick scan after restarting to confirm the threat is gone.

When your computer will not start or the scanner will not open

If your computer is so infected that it will not start normally or the scanner will not open, restart in Safe Mode with Networking. Safe Mode loads only the essential files Windows needs to run, which often lets the scanner work even when the virus is blocking it.

On Windows 10 or 11, hold the Shift key and click the Power button in the Start menu, then click Restart. Your computer will restart and show a menu. Click Troubleshoot, then Advanced options, then Startup Settings, then Restart. When the computer restarts again, press the number 4 or F4 to enter Safe Mode with Networking. Once in Safe Mode, open Windows Defender and run a scan the same way you normally would.

On Mac, restart and hold Command + S while the computer starts up. This opens Single-User Mode. Type fsck -fy and press Enter to check the hard drive, then type reboot and press Enter to restart normally. Then read and run Malwarebytes.

Scanning external drives and USB devices

Viruses can hide on external hard drives, USB flash drives, and SD cards. Plug the device into your computer, then open Windows Defender or Malwarebytes and choose Custom scan. Select the external drive from the list and click Scan now. The scanner will check only that device, which takes less time than a full system scan.

Always scan an external drive before opening files from it, especially if you received it from someone else or found it. If the scan finds a threat on the external drive, quarantine it. You can then safely delete the quarantined files or wipe the entire drive if you do not need what is on it.

Frequently Asked Questions

How often should I scan my computer?

Run a quick scan at least once a month as routine maintenance. If you read files frequently, visit unfamiliar websites, or open email attachments from people you do not know, scan every two weeks. If your computer suddenly runs slowly, crashes, or shows unexpected pop-up ads, scan when ready.

Is Windows Defender enough, or do I need a third-party antivirus?

Windows Defender is effective for most users and catches the majority of common threats. Third-party antivirus software like Norton, McAfee, or Kaspersky offers extra features like real-time monitoring and parental controls, but they cost money and can slow your computer down. Start with Windows Defender; if you feel you need more protection, add a paid option.

What if the scanner says a file is a threat but I know I need it?

Some legitimate programs trigger false alarms. If you are certain a file is safe, you can restore it from quarantine. Open Windows Defender, click Virus and threat protection, scroll down to Quarantine, and click Manage quarantine. Find the file, click it, and click Restore. Only do this if you trust the source of the file completely.

Can I scan my computer while it is running, or do I have to shut it down?

You can scan while using your computer. The scan runs in the background and does not require a restart to begin. However, the scan will run faster if you close other programs and avoid heavy tasks like video editing or gaming while it works.

What does quarantine mean?

Quarantine moves a suspicious file to a special isolated folder where it cannot run or spread to other files. The file stays on your computer but is locked away. You can delete it later, or restore it if you decide it was safe. Quarantine is safer than when ready deletion because it gives you a chance to check whether the file was important.