A PC that suddenly shows ads, redirects searches, or crawls under mystery CPU load usually has unwanted software on board — spyware, adware, or the “free download manager” that came bundled with something else. You can clean the vast majority of these infections with tools that are already free, in about an hour. Work through the steps in order; each one catches what the previous one misses.
Last checked: September 2026. Every tool below is free; no “PC cleaner” purchase is ever part of the fix.
Step 1: Full scan with Windows Security
- Open Windows Security (search it from Start) → Virus & threat protection.
- Choose Scan options → Full scan. Let it run to the end — an hour on a big drive is normal.
- Remove or quarantine everything it flags. Reboot.
Microsoft Defender is genuinely good now and already on every Windows 10/11 machine. Don’t install a second antivirus “for strength” — two real-time engines fight each other.
Step 2: Microsoft Defender Offline scan
Malware that hides while Windows runs can’t hide from a scan that runs before Windows does. In the same Scan options list, pick Microsoft Defender Offline scan. The PC reboots into a minimal environment, scans, and boots back — around 15 minutes. This is the step people skip, and it is the one that catches rootkit-class pests.
Step 3: A second opinion that doesn’t fight Defender
Install the free version of Malwarebytes — from malwarebytes.com only — and run one scan. Its free tier is an on-demand scanner, so it coexists with Defender, and its PUP (potentially unwanted program) detection is aggressive in exactly the category that ad-injectors live in. Decline the Premium trial toggle during install if you don’t want the 14-day nag. Uninstall it afterward or keep it for occasional scans — either is fine.
Step 4: Evict what installed itself as a normal app
- Open Settings → Apps → Installed apps and sort by install date.
- Look at everything installed around the day the problems started. Toolbars, “driver updaters”, “PC optimizers”, search assistants: uninstall them. If you don’t recognize a name, search it on your phone first — some entries are legitimate components.
Step 5: Clean the browser — where adware actually lives
- Extensions: in Chrome/Edge, open the extensions page and remove anything you didn’t knowingly install. This single step ends most “ads everywhere” cases.
- Notification spam: browser Settings → Site permissions → Notifications — remove sites you never meant to allow. Those “virus warnings” in the corner of the screen are usually just notification permissions, not infections.
- Hijacked search/start page: reset them in browser settings, or use the browser’s own “Reset settings” — it keeps bookmarks and passwords.
Step 6: Check what starts with Windows
Task Manager → Startup apps: disable entries you can’t account for. This doesn’t remove anything (safe to experiment) and it’s where leftovers announce themselves after the uninstall.
If it’s still infected
When something survives all of the above — reinfection on every reboot, disabled security settings, an antivirus that won’t start — stop trenching and use Windows’ nuclear option: Settings → System → Recovery → Reset this PC, with “Keep my files”. Modern Windows makes this a 30-minute operation, and it beats a weekend of manual registry archaeology.
Not getting reinfected
Almost every infection on this list arrives the same way: a download from a portal instead of the vendor. Get software from the developer’s own site, keep SmartScreen on, and treat “your PC has 2,417 problems” pop-ups as the advertisement they are. That habit is the entire prevention chapter — it’s the same reason we point people only to official sources in every guide on this site, from UniKey to Kodi.
Featured image: jaydeep_ / Wikimedia Commons, CC0