nfs 5 porsche 2000

i have nfs 5 porsche 2000/unleashed.
whenever i try to play it, it usually gets past the loading screen, and i can have one or two races, then it either restarts the computer for me or quits the game for me.
sometimes it just gets to the last loading screen, then it quits back to windows.
btw i have windowsxp, so maybe that's got something to do with it??????