anyone know why a computer might start restarting out of nowhere?

I am shocked

.
Its constant...i just reinstalled everything(yes the master drive and the slave is as is).
i am using XP pro.I am surfing the internet and all and it just restarts for no appearent(spelling?) reason.
this is very annoying and i dont know who to ask for this kind of question(everyone know i know pretty much knows nothing).
HEEEELP ! ?

lots of thanks in advance and all there used to be an Ati Rage Fury in this computer(surfing computer) and all i did was took this card out and put an AGP vante 8mb.
should i took for the driver of this card or something?
anyone?
