Another possibility is hardware failure…
check your event logs, system log for the event id ‘7’. That’s usually a bad block on the drive, and means drive replacement.
Wow, I’m Mr Doom and Gloom tonight, aren’t I? 😉
Was the windows key invalid at entry, or activation?
And.. Have to ask, legit copy of your OS? No need to say here, just something to consider.
AV is pretty much a must these days, and if my quad core 2008 era PC can run AV without slowdown any machine should be able to LOL 🙂
If you’ve seen issues after running malware scans though, it sounds like a malware system driver that’s been installed has been partially uninstalled by your anti malware, which perversely has broken the machine.
Have a look at bleepingcomputer.com, you may find some hints there.