Mine does this too. Its a few years old now and its been doing it from day one. Just restarts itself at random. Could be anytime, a day or two or a few minuits even between restarts, It could happen anytime. Really frustrating if working on something big. I havn't tried to get to the bottom of it. It really could be anything. I recently put winXP back on and have noticed that when it restarts it shows a blue screen with some writing on it but it only stays on screen for a milisecond before it goes so can't really make out what it all says..!! Just get used to it or send the pc back. Its harsh but there is very little one can do.
