Screen saver problems and artifacts

well when i let my pc sit for 10 min it goes into the screen saver mode which is nothing just black and then i get artifacts and the screen goes of and then turns on black with artifacts and it just does that. so i have to restart it next is when i use windows media center i play a movie and boom i get artifacts and the pc freezes and got to restart, and i kno its not my graphic card because it works with windows xp and everything was good.