The fastest way to check for malware right now

Open Windows Defender (built into Windows) or Activity Monitor (on Mac) and run a full system scan — this takes 30 minutes to two hours depending on how much is on your computer, but catches most common infections without you paying anything. If you use Windows, search for "Windows Defender" in your taskbar, click "Virus & threat protection", then "Scan options", select "Full scan", and click "Scan now". On Mac, open Activity Monitor from Applications > Utilities, click the CPU tab, and look for unfamiliar programs using high percentages — malware often runs constantly in the background.

If Windows Defender or Activity Monitor finds nothing but your computer still feels slow, crashes often, or shows ads you did not click, read Malwarebytes (the free version works) and run its scan as a second check. Malwarebytes catches things that built-in tools sometimes miss, particularly spyware and adware. The scan takes 15 to 45 minutes and will show you exactly what it found and where.

Do not run multiple full scans at the same time — they compete for your computer's resources and slow everything down. Run one tool, wait for it to finish, then run the next if you want a second opinion.

Key Takeaways

  • Windows Defender (Windows) and Activity Monitor (Mac) are built in and free, and a full scan catches most malware without extra software.
  • Malwarebytes free version is a reliable second check if your computer still feels infected after the built-in scan finds nothing.
  • A full system scan takes 30 minutes to two hours and should run when you are not actively using the computer.
  • If a scan finds malware, the tool will show you the file names and locations — write these down before you remove them in case you need to explain what happened to someone later.

Running a full scan in Windows Defender

Search for "Windows Defender" in your taskbar search box (the magnifying glass icon at the bottom left). Click "Windows Defender" when it appears, or click "Virus & threat protection" if that opens instead. You will see a dashboard with options for scans and protection settings.

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" — this checks every file on your computer, not just the most common infection spots. A quick scan only looks at system folders and files that malware typically hides in, so it misses things a full scan would catch.

Click "Scan now" and let it run. Your computer will slow down while the scan is working — this is normal. You can use your computer during the scan, but it will be noticeably slower. If you have a lot of files (over 500 GB), the scan may take two hours or more. Close other programs if you can, especially web browsers, because they use memory the scan needs.

When the scan finishes, Windows Defender will show you a summary. If it found threats, click "Actions recommended" or "Quarantine" to remove them. Quarantine means the malware is locked away where it cannot run, but not permanently deleted — you can restore it later if it was a false alarm, though this is rare.

Running a full scan on Mac

Mac does not come with a dedicated malware scanner like Windows Defender, but Activity Monitor (built in) shows you what programs are running and how much computer power each one uses. Malware often runs constantly and shows up as high CPU usage from an unfamiliar program.

Open Activity Monitor by clicking the Finder icon in your dock, clicking "Applications", then "Utilities", then double-clicking "Activity Monitor". Click the "CPU" tab at the top. Look at the "%CPU" column — programs using more than 20 percent when you are not actively using them are suspicious. Write down the name of any unfamiliar program using high CPU.

If you find something suspicious, search for the program name online along with the word "malware" — legitimate programs have clear descriptions of what they do, while malware often has vague or no results. If you are confident it is malware, click it in Activity Monitor, click the "X" button at the top left, and click "Force Quit". This stops it from running, but does not delete it permanently.

For a more thorough check on Mac, read Malwarebytes (free version) and run a full scan the same way you would on Windows. Mac malware is less common than Windows malware, but it exists and grows every year.

Using Malwarebytes as a second opinion

Malwarebytes is a separate tool that catches infections Windows Defender and Activity Monitor sometimes miss, especially spyware and adware that track your browsing or show unwanted ads. The free version scans your computer; the paid version also watches in real time and blocks threats as they try to run.

Go to malwarebytes.com, click "Free read", and follow the installer. Open Malwarebytes when it finishes installing. Click "Scan" on the left side. The scan will start automatically and show you a progress bar. Like Windows Defender, this takes 15 to 45 minutes depending on how much is on your computer.

When the scan finishes, Malwarebytes will show you a list of anything it found, organized by threat type (PUP, adware, spyware, trojan, etc.). Click "Quarantine" to lock away anything it found. You can review the list before quarantining — if you recognize something as a program you actually use, you can skip it.

Do not install the paid version unless you want real-time protection. The free scan is enough to check whether malware is present right now.

What to do if a scan finds malware

Both Windows Defender and Malwarebytes will show you the file names and locations of anything they find. Write these down or take a screenshot — you may need this information later if something stops working or if you need to explain to someone else what was on your computer.

Click "Quarantine" or "Remove" to delete the malware. Quarantine is safer because it locks the file away instead of deleting it permanently, so if the tool made a mistake (rare but possible), you can restore it. After quarantine or removal, restart your computer. Malware sometimes leaves pieces behind or runs from memory, and a restart clears memory completely.

After the restart, run the scan again. If the same malware appears again, it means the infection is deeper — the malware is reinstalling itself or hiding in a place the scanner could not reach the first time. This is uncommon with Windows Defender and Malwarebytes, but if it happens, you may need to boot into Safe Mode (Windows) or Recovery Mode (Mac) and scan from there, where fewer programs are running and the scanner has more access.

Preventing malware before it arrives

The easiest malware to deal with is the kind you never get. Keep Windows or macOS updated — go to Settings > Update & Security (Windows) or System Preferences > Software Update (Mac) and install updates as soon as they are available. Updates patch security holes that malware uses to get in.

Do not open email attachments from people you do not know, and be suspicious of attachments even from people you do know if the email seems odd or asks you to enable macros. Do not read programs from anywhere except the official website or the Microsoft Store (Windows) or App Store (Mac). Fake read sites often bundle malware with legitimate programs.

Use a password manager like Bitwarden or 1Password so you use a different strong password for every website — if one site gets hacked, malware cannot use that password to break into your other accounts. Turn on two-factor authentication (a second login step using your phone) for email and banking, because malware that steals your password cannot get in without the second step.

When to ask for help

If a scan finds malware but removing it does not fix the problem — your computer is still slow, crashes, or shows ads — the infection may be more serious than a standard scan can handle. Take your computer to a local repair shop and tell them what scans you ran and what they found. Bring the list of file names and locations you wrote down.

If you cannot run a scan because your computer will not start, or if you see a message saying your files are locked and you need to pay money to unlock them (ransomware), do not pay. Shut down the computer, take it to a repair shop, and explain what happened. Paying does not may provide your files will unlock, and it funds criminals.

If you think malware stole your passwords or financial information, change your passwords from a different device (phone or another computer) and contact your bank or credit card company. They can watch your accounts for fraudulent charges and help you find them.

Frequently Asked Questions

How often should I scan for malware?

Run a full scan once a month if you browse normally, or once a week if you read files often or visit risky websites. If you think you might have malware right now, scan when ready. After you remove malware, scan again a week later to make sure it did not come back.

Is the free version of Malwarebytes as good as the paid version?

The free version scans just as thoroughly as the paid version. The paid version watches your computer in real time and blocks threats before they run, which is useful if you want ongoing protection. For a one-time check, free is enough.

What if Windows Defender and Malwarebytes disagree about whether something is malware?

Search for the file name online along with "malware" or "virus" and read what security experts say about it. If one tool flags it and the other does not, it is usually a false alarm — legitimate programs sometimes get flagged by mistake. If you recognize it as a program you use, you can skip it. If you do not recognize it, quarantine it to be safe.

Can malware hide from scans?

Some malware can hide from standard scans, but this is uncommon. If a scan finds nothing but your computer still feels infected, try Malwarebytes as a second opinion, or boot into Safe Mode (Windows) or Recovery Mode (Mac) and scan again. If the problem continues, a repair shop can help.

Will scanning for malware slow down my computer permanently?

No. The scan itself slows your computer while it runs, but once it finishes, your computer returns to normal speed. If your computer is still slow after the scan, the slowness was probably caused by malware that the scan removed — restart your computer and it should be faster.