I don't think it's the power supply as well. I mean it's an old computer and whatever video he installed the computer got passed post so chances are it's fine.
It can be Hd or just a corrupt windows copy. Did you try the windows disk to fix? That actually works more times then not.
Do yourself a favor and burn yourself either
http://www.ubuntu.com/
http://knoppix.com/
Either have a bootable cd that you can check to see if your video, ram cd, floppy drive works or not. Meaning if your comp boots chances are most of the hardware are fine.