Quote from gnome:
It's a good idea to have an extra hard drive in any event. You can use Acronis to make a clone.
I think a virus can infect the MBR, so reformatting yours if your MBR is infected might not clear the problem.
If you get a 2nd hard drive, format and install WinXP. After you've got it all loaded, run a Low Level Format utility on your present hard drive. That should erase the MBR. If that doesn't clear the problem, then as earlier poster said... likely some other issue.
You can run MemTest to check the RAM... but it could be heat issues.