I Assume you've done a virus scan and malware scan (Avira antivirus
and Malwarebytes Anti-malware
are ones I use) to make sure you've no "culprits" infecting your computer. You might have to do these scans in SAFE-MODE if Normal mode prevents you from using the computer for any length of time.
If you can rule malware out then you can try other things.
Since the drive functioning is "sporadic" I'd guess either an issue with the motherboard or the controller in the drive itself.
You can try is plugging the hard drive into another IDE port (if you're using an IDE drive) or another SATA port (if you're using a SATA drive). I've personally had ports fail on me and had to resort to using a different port until I could upgrade to a new motherboard.
Scanning the drive for errors: Even if the drive checks out, the controller might be failing, which would cause problems. If you've a spare drive lying around, you can install it and do an install of Windows without activating it and see how the performance is.
Temporarily removing cards from the computer might help to narrow down the problem, i.e. remove the video card and use the built-in VGA connector (if you have one -- or use another video card). Remove additional cards, like sound, TV cards, etc.
Try different memory configurations, i.e. if you have to memory cards, use one at a time and see what happens.
Hope this helps,