Windows restarts and shuts down by itself

Hello,
Fortunately its not my comp that is screwy. My buddy and I both have the same exact pcs. He had shutdown his pc for 2 days when he came back and turned it on the computer would bring him to his user page (choose which user to log on). he would click his and then it would bring to desktop then a blue screen would come up with an error reading. he was unable to get the error reading because it would restart in less than 2 seconds. He has tried to run it in safe mode and it will not let him at all to get in. he tried last known config, safe mode with command, everything . it does the same thing. operating system is windows xp SP2. ive searched through google to try to find and it basically says all the same stuff. can anyone please help, how can he fix this ? thank you