i assume you have run scans for a virus and also scanned for spyware and found nothing. Windows XP normally reboots without warning when a device driver crashes. you will have to think back to what you might have installed just before these crashes started happening and decide if one of them might be the cause. games are usually the ones that could install device drivers without really telling you. once your system is crashing regularly, it will crash more and more often until you find the cause and fix it or do a clean install.
Herb...
Hi..
I need some serious help. I have a compaq presario that's a couple of years old. recently a friend of mine upgraded it from win 98 to windows xp .It was working fine for a month or 2 until at the beggining of March the computer after a while just turns off out of nowhere and reboots. before it was rare only after more than an hour. Now it happens all the time . I'll be on it for 15-20 minutes when it just shuts down with no warning and the reboots. Then after another 20 minutes shuts down again and reboots. I dont know what could be causing the problem . Can anybody help? has this happened to anyone else?
if you can please steer me in the right direction i would appreciate it..
thanks,

Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic