What malware is and why your computer needs scanning

Malware is software designed to harm your computer, steal your information, or use your machine without your permission. It includes viruses, spyware, ransomware, and adware. Unlike a virus you catch from another person, malware spreads through downloads, email attachments, infected websites, and USB drives. You cannot see it working, which is why scanning is the only way to know if your computer has it.

A malware scan searches your entire hard drive, looking for known malware signatures and suspicious behavior patterns. Windows and Mac both include built-in scanning tools that work without paying anything. Third-party scanners exist too, but the built-in tools catch the vast majority of common threats and are sufficient for most people.

Scanning takes time — anywhere from 15 minutes to several hours depending on how much is on your drive and which tool you use. Plan to run a scan when you will not need your computer for other tasks, or run it overnight.

Key Takeaways

  • Windows Defender (built into Windows) and Malwarebytes are the two most common tools for finding malware, and you can run both without paying.
  • A full system scan checks every file on your computer and takes longer than a quick scan, but catches more threats.
  • If a scan finds malware, the tool will quarantine it (lock it away so it cannot run) and give you the option to remove it permanently.
  • After removing malware, restart your computer to make sure the infection is fully cleared and any remaining pieces are cleaned up.
  • Running a scan once a month or after visiting unfamiliar websites keeps your computer safer without slowing it down between scans.

How to run a malware scan on Windows

Windows includes a built-in scanner called Windows Defender (also called Windows Security). It runs in the background automatically, but you can also run a manual scan whenever you want. Open Windows Security by typing "Windows Security" into the search box at the bottom left of your screen and clicking the result.

Once Windows Security opens, click Virus & threat protection on the left side. You will see a section called "Current threats" with a button that says Scan options. Click that button. You will see four choices: Quick scan, Full scan, Custom scan, and Microsoft Defender Offline scan. For a thorough check, choose Full scan — it takes longer but searches every file on your computer. Click Scan now and let it run. Do not shut down your computer while it is scanning.

When the scan finishes, Windows Defender will show you what it found. If it detected malware, it will ask whether you want to remove it. Click Remove or Quarantine (both prevent the malware from running). After removal, restart your computer to complete the cleaning process.

How to run a malware scan on Mac

Mac includes a built-in malware scanner called Xprotect, but it only runs automatically when you open files. To run a manual scan, you need a third-party tool. Malwarebytes for Mac is free and widely used. Go to malwarebytes.com, click Download, and follow the installer. Once installed, open Malwarebytes and click the Scan button.

Malwarebytes will run a scan of your entire system. Like Windows Defender, it takes time — usually 20 minutes to an hour depending on your drive size. When it finishes, it will show you what it found. If malware is detected, click Remove and restart your Mac to complete the process.

If you prefer not to install anything, you can also restart your Mac in Safe Mode (which disables most third-party software and makes malware harder to hide) and then run a scan with Malwarebytes or another tool. To restart in Safe Mode, shut down your Mac, turn it back on, and hold the Shift key until you see the login screen.

Understanding scan results and what quarantine means

When a scan finishes, you will see a report showing how many threats were found, if any. The report lists each piece of malware by name and location on your drive. Quarantine means the scanning tool has locked the malware away in a separate folder so it cannot run or spread. Quarantined files are harmless — they cannot affect your computer anymore.

Most of the time, you should remove quarantined malware permanently rather than just leaving it quarantined. To do this, go back into your scanning tool (Windows Defender or Malwarebytes), find the quarantine section, and select Remove or Delete. This erases the malware completely from your drive.

Occasionally a scan will flag a file that is actually legitimate — this is called a false positive. If you recognize the file name and know it came from a trusted source, you can tell the scanner to ignore it. But if you are unsure, it is safer to let the scanner remove it. You can always reinstall legitimate software if needed.

What to do if your computer is heavily infected

If a scan finds dozens of threats or your computer is running extremely slowly, you may have a serious infection. In this case, run the scan again and remove everything it finds. Then restart your computer and run another scan — sometimes malware hides pieces of itself that only show up after a restart.

If malware keeps coming back after you remove it, or if your computer will not start up normally, you may need to use a more aggressive tool. Windows includes Microsoft Defender Offline scan, which scans your computer before Windows even loads. This catches malware that hides from the regular Windows Defender scan. To use it, open Windows Security, go to Virus & threat protection, click Scan options, select Microsoft Defender Offline scan, and click Scan now. Your computer will restart and scan before Windows loads.

If offline scanning does not work, or if you are not comfortable doing this yourself, take your computer to a local repair shop. They have tools and experience for severe infections that home scanning cannot fix.

Preventing malware between scans

Scanning finds malware that is already on your computer, but prevention stops it from getting there in the first place. Do not download files from unfamiliar websites, do not open email attachments from people you do not know, and do not click links in suspicious emails or text messages. These are the three most common ways malware spreads.

Keep your operating system and software updated. Windows and Mac both release security patches regularly — these fix holes that malware uses to get in. Turn on automatic updates so you do not have to remember to do it manually. You can check for updates by going to Settings on Windows or System Preferences on Mac and looking for an Updates section.

Run a full scan once a month as routine maintenance, or more often if you download files frequently or visit unfamiliar websites. A monthly scan takes a few hours but catches problems before they become serious.

Frequently Asked Questions

Can malware hide from Windows Defender?

Some malware is designed to avoid detection, but Windows Defender catches the vast majority of common threats. If you suspect your computer has malware that Windows Defender missed, run Malwarebytes as a second opinion — the two tools use different detection methods and often catch different things. Running both gives you much better coverage.

Will scanning slow down my computer?

A scan uses a lot of processing power while it is running, so your computer will be slower during the scan. Once the scan finishes, your computer returns to normal speed. If your computer is slow even when no scan is running, malware might be the cause — run a full scan to check.

What if I accidentally delete a file I need?

If you removed a file that turned out to be legitimate, you can restore it from your backup if you have one. Windows includes a backup feature called File History; Mac has Time Machine. If you do not have a backup and the file is important, a data recovery service can sometimes retrieve it, though this costs money.

Do I need to pay for malware scanning?

No. Windows Defender is free and built into Windows. Malwarebytes has a free version that scans and removes malware. Paid versions of these tools add real-time monitoring (scanning files as you download them), but the free versions are sufficient for regular scanning.

How often should I scan my computer?

Once a month is a good routine for most people. If you download files frequently, visit unfamiliar websites, or share your computer with others, scan every two weeks. If you notice your computer running slowly or behaving strangely, scan immediately.