First of all could you post some information detailing the environment around your pc. Such as, are you using a surge protector or UPS, is there anything on the same electrical ciruit as PC that draws alot of power at random times (refrigerator, air conditioner),have you looked in event viewer for any recurring errors that coincide with the restarts. With this information it will be much easier to identify and rectify your problem.
This problem is driving me nuts! Running XP sp2, 2.4 ghz, 1 gb ram, 180 gb HD, Abit MB, ATI 9600 All-In-Wonder graphics card. It has been doing this since the first day it came home. If I do a complete clean up every day, defragment, clean up HD, run Registry First Aid, run SpyBot, run Spy Sweeper, it will maybe work ok for that day. It has been back to the shop that built it. They have ruled out a virus, tested the power supply and of course it did not do it while it was in there! They gave me a new power cord. That did not help any! Any clues out there please ?