It's sounding as if this is probably a hardware issue, rather than a software issue. But for those interested, I'd like to share this tip that has solved a lot of software issues for me over the years. I use ERUNT to backup the registry from time to time. I have made a Bart PE boot CD, and included antivirus software on it. The Bart PE CD, lets me boot to a clean Windows XP from the CD. I can use it to restore the ERUNT registry backup, which I find solves most problems where the PC has stopped working correctly because of software problems. I can also use it to do a virus scan of the HDD, even if the native Windows won't boot. Also booting to Bart PE can give you an indication of whether your problem is hardware or software related, if the hardware works under Bart PE, then the problem is most likely with your software, this is not 100% proof, your hardware may be under more stress when booted from the HDD, or it may not work under PE for lack odf drivers, but it is some indication.

ERUNT, registry backup.
http://www.larshederer.homepage.t-online.de/erunt/

Bart's Preinstalled Environment (BartPE) bootable live windows CD/DVD.
http://www.nu2.nu/pebuilder/

Dr.Web CureIt, free antivirus scan.
http://www.freedrweb.com/cureit/?lng=en