A spontaneous reboot often means your computer is tossing a Blue Screen of Death but is set to not display it. BSODs are generally caused by hardware or driver problems, and the error message it shows can help pin down exactly what the issue is.

To have your computer display BSODs instead of rebooting immediately, go to Control Panel->System Properties->Advanced Tab->Startup and Recovery Settings and uncheck 'automatically restart'.

The next time it crashes, you may get a BSOD similar to the one below (not all highlighted parts may be present):


Write down the information in the highlighted areas before turning your computer off. The information there can then be researched online to see if others have experienced similar problems, and what the cause was. You may also get a 'windows has recovered from a serious error' message when you turn your computer back on--if you do this, you should agree to send the report to microsoft. You'll then be forwarded to information about that error on Microsoft's knowledgebase, if they have any.

Just in case this helps anyone, here's an update.  Knock on wood, I think I found something that has improved the stability of Demigod on my system.  After reading about some stability issues some users had with Gigabyte P55 chipset motherboards and quadcore i5/i7 CPUs, which I have, I decided to try two BIOS changes that I found in some forums relating to Gigabyte issues.

1.  Set PCE-E speed to 100 mhz, instead of "auto"

2.  Disable C3/C6/C7 CPU support.   (Unfortunately this kills the automatic/dynamic speed changes on multicore i7 CPUs)

After these changes, I've played Demigod for at least 6 hours without a crash (not all in one sitting).  Where before it would tend to crash in less than an hour.

Still don't know why Demigod is the only software that I had crash issues with, when my system was stable in Prime95, Furmark GPU burn-in tests, Crysis, etc.  CPU and GPU temperatures were and are OK.