Demo causes computer to shut down!

[pwinant]

I've been playing computer games for decades but this is one I've NEVER seen...

I can start the demo fine, select a new character (paladin) and go through the intro video, but as soon it loads the game up (after it "fills up" the circle showing the loading progress) my computer turns off. I don't mean it does some kind of Windows shutdown, it's literally like somebody flipped off the power switch! I thought maybe the power supply had a problem so I rebooted and tried running the demo again and it happened again in the exact same spot. I'm fairly computer-savvy but I didn't even know it was possible for any application to do that to a computer.

I am running Windows XP Professional. I have a GeForce 8800 GTS and a SoundBlaster X-Fi. Can anyone explain why this might be happening or suggest any way to fix or debug it?

Thanks!
Phil