Can you identify the date when the problem started?
If so, you might try to see if you can boot into Safe Mode and then use System Restore to roll back to a date/time before the problem started.
In case you're not sure how to boot into Safe Mode:
1. Shut down your computer.
2. Start it up, then immediately start tapping the F8 key.
3. When the Windows Advanced Options Menu appears, select Safe Mode (I shouldn't worry about networking, etc.).
After some weird goings-on on the screen, you'll get a crude Windows XP desktop, with the words 'Safe Mode' in all four corners of the screen.
Now run System Restore, as suggested above.
If your problem is Windows related, this might work. If it's BIOS or hardware related, it probably won't. But it's worth a try.