My computer keeps freezing!!

Hi there
Can anyone help me? my computer freezes after a few min of not moving the mouse around or anything, if I leave my computer for a short ammount of time it freezes. No message comes up, and it happends on whatever screen I happen to be on. nothing works except just shutting it off by holding the power button in.

Anyone have any idea's on what it could be? I've done scan's and they all come up clean.

Please help me, I don't know what to do, this is a brand new computer and it's driving me crazy!!!

Thanks