Random shut down

Hey, so I've been searching around the forums and can't find an exact answer to this problem.

I recently threw together a computer:
ASUS mobo M5A97 evo
AMD FX-6100
NVIDIA GeForce GTX 550 Ti
GSKILL Ripjaws 1066
WD Blue 500 gig 7200rpm
500w Cooler Master psu
In an Apecvia X-gear case
Running Windows 7

So I've had this setup for maybe a month now, and I've been having problems with the computer shutting down randomly. The CPU runs usually about 48-50 C and everything else seems to be running cool enough, the highest being the GPU at 58-60 C sometimes. It seems hot, but I've heard it's normal. I've done regular virus/malware scans with Norton, and I'm pretty sure it's not lack of power.

Any advice would be greatly appreciated, thanks!