being having this random crashes with wow , oblivion , the witcher. cod4 seems to work fine though. brand new comp , got it like 4 weeks ago and still cant fix this. if you are know a lot about comps im begging ya to pleae help me out. i can give you more info if you need it just let me know.
hard locks are usually a result of either overheating, bad memory, bad mother board, bad power suplly, bad video card or a combination of any of those.
start by making sure the fan(s) are running and none of the vents on the case are blocked keeping air from getting in or out. if that don't work start swapping components to figure out what is causing it...different memory sticks, different power supply, etc.
the bad news is hard locks very difficult to diagnose if you dont have extra components. but since you said you computer is only 4 weeks old, take it back to where you got it and get a replacement.