Houston, we have a problem !!! (well, sister does in fact lol)

Why does her computer keep freezing ? No matter what its doing, in fact it can just be sat on the desktop with nothing else running, and it stops, no nothing, not even able to move the cursor

I even installed a new system on it yesterday, and it still does it which obviously points to hardware ?

Any ideas anyone ? The only one i can thing of is maybe its over heating