Just built my first gaming PC and am having problems running software.

Last night I finished building my first gaming PC (I've gamed on a laptop up until now)

I'm sure I'll get asked this, so here is what I have:

Motherboard: ASUS P8Z77-M PRO LGA 1155 Intel Z77 HDMI SATA 6Gb/s USB 3.0 Micro ATX Intel

GPU: Galaxy GeForce GTX 670

CPU: Core i5 3570K 3.4GHz LGA 1155

PSU: Rosewill CAPSTONE Series CAPSTONE-750

Hard Drive: Seagate Barracuda Green ST2000DL003 2TB 5900 RPM 64MB Cache SATA 6.0Gb/s

RAM: CORSAIR Vengeance 8GB (2 x 4GB) 240-Pin DDR3

The build itself went fine, other than the usual little issues here and there. I installed all the neccesary drivers.

This morning, at first testing software went fine. I started with Dear Esther, something system intensive, but far from bleeding edge. No problems, played the entire game.

The problems started when I moved to something a bit more intensive: The Witcher 2. I was able to start the game get fine, the opening cinematic worked fine. However, as soon as I loaded into a game, within five seconds of gaining control of my character, my entire system shut off then restarted. It did this another few times, as I tried tweaking settings, increasing max temperature and voltage, etc.

After having no luck with The Witcher, I tried Skyrim, hoping it was just some software issue that I could fix later. Skyrim wasn't quite as bad, but I still can't get in game. Again, I can make it through the opening menu and what not. Then it loads for a few seconds, sounds shift to the usual sounds of the world, video goes black... and I crash straight to desktop, no warning, no error message, nothing.

I am fresh out of ideas. I've checked that everything is plugged in correctly, I've tweaked every setting I can think to tweak.

At this point, any suggestions whatsoever would be much appreciated.