Sounds like some issue I encountered with a defective BIOS. Check with the maker for BIOS updates if you can't restart with the USB stick plugged in.
Whenever I'm playing a game, mostly Civ 5, but also Garry's Mod, Far Cry 3, and many others, my computer will freeze, and the only thing I can do is turn it off with the power button.
There's nothing wrong with my HDD and I ran Memtest, but nothing showed up.
It's worth noting that any buzzing I hear through my headset, which is probably from the GPU, stops entirely when my comp freezes.
I also need to take out my Cisco wireless USB adapter and plug it back in to get it working properly when the computer is back up again.
Any and all help is appreciated.
GeForce GTX 560
2 x Ripjaws F3-10666CL9D-4GBRL
WD Caviar Green 500GB