Computer restarts.

My computer restarts when I open games that are full screen. For those people who don't know, it's like one of those games you see in ads like Grand fantasia, Maple story and some other shit like that.
I don't know what's going on and I really need to play a game. It looks great.
I'm using windows xp.

Ookay standard question.
How much memory, what kind of graphic card, how old is the graphic card drivers?It could be anything from overheating to old graphic card drivers, so little more info please :)