two things:
hardware failures eg RAM (usually this results in more random results), Hard drive dying (usually gets worse over time), heat - might be ok if you leave it off for a long time then try again. CPU/chipset/mainbord - like RAM.
Software failure - Windows is deaded, and the most recent restore point is also probably bad. From what you describe, this is most likely. Try a manual restore from an older restore point. Instructions for this are around the webs somewhere. Pain in the arse to do. Also, can try to do an 'upgrade' install where you install XP over the top of the existing system, and then re-install drivers. I /think/ this requires a windows retail disk (not oem).
( Sextant goes to him is emotionally erect, on this day, Sat 12 Dec 2009, 14:40, Reply)