Why did another malware detection show up a day after I cleaned my PC?

Not necessarily. A second detection could be a leftover file, something newly downloaded, or a different item the first scan missed. Check the detection name, file path, and scan dates in the Malwarebytes reports before assuming it came back.

Update Malwarebytes and run another full scan. Also check Windows Security for updates and run a scan there. If the same file or detection keeps returning, that is more useful evidence than simply seeing two different alerts.

Look at what was detected before deciding how drastic to get. Adware and a serious malware infection aren't necessarily the same thing, and a scanner's label and file location can help explain what needs attention.

Check recently installed apps and browser extensions for anything you don't recognize. Avoid opening suspicious files again, and don't restore anything from quarantine unless you've established it was a false positive.

If detections keep returning or you can't trust the installation, a clean Windows install is an option. Back up your personal files first, then check that the backup is readable. Be cautious about bringing old programs or suspicious downloads back onto the fresh system.

Installing Windows can erase data, especially if you delete partitions. Double-check which drive you're working on and make sure anything you need is backed up before proceeding.