A computer that got slow overnight, runs its fan for no reason, and throws pop-ups you did not summon is often carrying a passenger. A slow computer can be caused by background malware quietly using your CPU, your network, and your patience. Sometimes it is mining cryptocurrency for a stranger. Sometimes it is just adware that hijacked your browser. Either way, you can usually clear it yourself for free, and the steps are the same.
One rule sits above everything else in this guide, so I will say it now and again later: only download security tools from the vendor’s own website. The single most common way people make an infection worse is downloading a “cleaner” from an ad or a search result that is itself the malware. If malware is only one suspect and the machine is generally sluggish, work the full how to fix a slow computer checklist too, because a full drive and a dying hard disk look a lot like an infection from the outside.
Signs malware is the cause
How do you know it is malware and not just a full disk? You look for the fingerprints. Any one of these on its own might be innocent. Several together point hard at an infection.
- Pop-up ads appear on the desktop or in the browser when you are not even browsing.
- Your browser homepage or default search engine changed by itself, and changing it back does not stick.
- New toolbars or extensions you never installed.
- The fan runs and the machine is hot while idle, and Task Manager shows a process you do not recognize eating the CPU.
- Redirects: you click a normal link and land somewhere else.
- A full-screen “your PC is infected, call this number” warning. That one is a scam by definition. Microsoft never puts a phone number on an error. Do not call it, do not let anyone remote in.
Open Task Manager with Ctrl+Shift+Esc and sort the Processes tab by CPU. Something using 40 percent while you do nothing, with a name that is gibberish or pretends to be a system file in the wrong folder, is your prime suspect. Do not just End task and call it done; that kills the symptom, not the cause. Note the name and move to the scanners.
Scanning with Windows Security and Malwarebytes (two scanners)
Run two scanners, not one. No single engine catches everything, and the second one catches what the first missed. Start with the one already built into Windows.
Open the Windows Security app (press Start and type Windows Security), go to Virus & threat protection, then Scan options. Pick Full scan and start it. A full scan reads every file on the drive and takes a while, so let it run. If it finds something, let it quarantine or remove it, then reboot and scan again to confirm it is gone.
Now the second opinion. Malwarebytes Free is the tool most technicians reach for as a second scanner; it runs on demand, finds adware and the “potentially unwanted programs” that regular antivirus often shrugs at, and it does not fight with Windows Security. Download it only from the official page, malwarebytes.com/mwb-download, and nowhere else. Install it, update its database, and run a full scan. Quarantine what it flags.
I will repeat the rule because this is exactly where people fall down. Type the vendor’s address into the browser yourself. Do not search “malwarebytes download” and click the top result, because paid ads and copycat sites sit above the real one and serve poisoned installers. Same goes for any tool: the maker’s own domain, typed by hand.
Cleaning the browser (extensions, reset)
Open your browser’s extensions page. A huge share of “malware” is really a browser hijack: a rogue extension that injects ads, changes your search engine, and rides along no matter how many times you scan the disk. The scanners do not always pull these, so you do it by hand.
- In Chrome go to
chrome://extensions; in Edge go toedge://extensions. Remove anything you do not recognize or did not deliberately install. When in doubt, remove it. You can always add back the one you actually use. - Check your search engine and homepage. In Chrome that is Settings, then Search engine, and On startup. Set them back to what you want. If they snap back to something strange, an extension or app is still doing it.
- If the hijack will not let go, reset the browser. Chrome: Settings, then Reset settings, then “Restore settings to their original defaults.” Edge: Settings, then Reset settings. This wipes extensions, pinned tabs, and start pages back to default without touching your bookmarks or saved passwords.
While you are cleaning, look at what launches with Windows. A lot of adware installs a startup entry so it comes right back after a reboot. Walk through how to disable startup programs on Windows 11 and kill anything you did not put there.
Safe Mode scanning
Some junk hides while Windows is fully running. It locks its own files, restarts itself the instant you kill it, and slips past a normal scan. Safe Mode is how you corner it, because Safe Mode starts Windows with only the bare drivers and services, so most malware never loads and cannot defend itself.
To get there in Windows 11: open Settings, go to System, then Recovery, and under Advanced startup click Restart now. When the blue menu appears, choose Troubleshoot, then Advanced options, then Startup Settings, then Restart. On the list that follows, press 4 for Safe Mode or 5 for Safe Mode with Networking (pick 5 if you still need to update a scanner). Run your full scans again from here.
There is a stronger version for the stubborn cases. Windows Security has a Microsoft Defender Offline scan under the same Scan options list. Choose it and the machine reboots into a tiny scanning environment that runs before Windows loads at all, which catches rootkits and boot-level infections that even Safe Mode cannot reach. It takes about fifteen minutes and it is worth the wait when something keeps coming back.
When to wipe and reinstall
Sometimes the honest answer is to start over. If two scanners plus an offline scan keep finding the same thing, if the infection touched banking or passwords, or if the machine simply never feels trustworthy again, a clean reinstall is faster and safer than a month of chasing ghosts. Wiping the drive removes the infection with certainty. Everything else is a probability.
Back up your personal files first (documents, photos, anything irreplaceable) to an external drive, and scan that backup on a clean machine before you trust it. Then in Windows 11 go to Settings, System, Recovery, and choose Reset this PC. You can keep files or remove everything; for a real infection, remove everything and let it reinstall Windows fresh. If you also want the speed jump while the machine is open, this is a natural moment to upgrade an HDD to an SSD and do a clean install onto the new drive.
After any reinstall, change your important passwords from a different, clean device: email first, then banking, then everything that mattered. If the malware was a keylogger, your old passwords are already somewhere they should not be.
When to stop and pay a pro
Adware and browser hijacks are beginner cleanups; anyone can do them with the steps above. Draw the line at three things. Ransomware, where your files are encrypted and a note demands payment, is not a DIY job; disconnect the machine from the network and get help, and never pay first without advice. Anything involving your bank account or a business full of client data deserves a professional, because certainty is worth the fee. And if the offline scan keeps finding a rootkit that will not clear, hand it over. A shop wipes and rebuilds these all day, and the peace of mind is cheap next to a drained account.
The repair ticket
Symptom: The computer slowed down suddenly, shows pop-ups and browser redirects, runs hot and busy while idle, and a strange process is eating the CPU.
Cause: Background malware, most often adware or a browser hijacker, sometimes a coin miner or something nastier, consuming resources and redirecting your traffic.
Fix: Scan with Windows Security and then Malwarebytes (both downloaded only from the vendor’s own site), remove rogue browser extensions and reset the browser, rescan in Safe Mode or run a Defender Offline scan for anything that hides, and wipe and reinstall Windows if it keeps coming back or touched your money. Change passwords from a clean device afterward.