Comp randomly shutting down

So I was playing WOW and was in the middle of a raid, and boom, my computer shut down.  No warning or anything.  It happened before while playing Half-Life 2.  Any Ideas?

could also be a memory problem, since those do almost always produce bluescreens with stop messages ( or reboots in your case ). just because a comp crashes it doesnt automatically have to be heat related, i still dont get why people always suggest this first. the psu being too weak is also a very rare case. id say its your ram, especially because the source engine and wow are both very memory-intensive, and faulty ram or ram timing problems occour VERY often.
just run memtest86 ( use google if you dont have it yet ) for a few hours, if nothing happens, you can worry about heat or whatever.
also, get motherboard monitor and check your temps WHILE playing. entering your bios after the crash and reading the temps isnt really helpful because CPUs and GPUs cool down very fast when not under load. my 7800gt takes about 5 seconds to cool down from 70 to 40 °C.