computer restarting when playing mpeg-4 video..help please?

my computer restarts whenever i try to play an mpeg 4 video.

(im not a computer noob and i cant figure this one out).

when i click on an mpeg4 in itunes my computer immediatly shuts down (just like if the plug was pulled or on button was held for like 4 sec, not a reg shut down)

it also happens when streaming mpeg4. i would think this is a codec problem but i have quicktime installed so there shouldnt be any issues.

i only realized this when i accidently clicked on a video in ituens and my computer went blank. ARGHHH

hopefully someone has had this problem and can help me out