mysterious restarts

I’ve got a computer that i don’t know where to go with. Once I power it up, it will run up to a couple of hours before resetting itself as if some little gremlin hit the reset button, and then resets again a few times before it gets the chance to start windows. I’ve checked the event logs under windows xp and it’s not picking up any errors and I’ve eliminated reset switch itself because i’ve unplugged it and this problem is still occuring. Does anyone know what the cause of this problem is?