Some freezing/crashing/slowdown issues are being caused by an audio conflict.  The developers are aware of the issue and are working on a solution for it. 

There are a few things you could try that have worked for many people on the forums.

First, make sure your audio drivers are completely up to date.

Second, go to the audio settings in the game and turn off the Reverb option.

Third, go to the Windows Control Panel and open your audio/sound settings and change the configuration to 'Stereo'.

Lastly, if none of the other options helped, make a shortcut to Demigod on your desktop and open it's properties.  At the end of the 'Target' line add a space and "/nosound" (without the quotes).  This will load the game with all sound rendering turned off(playing the game without sound really isn't as fun, but it will make the game playable until the issue is resolved).

If your computer is completely shutting down then you are most likely looking at a heat or power issue. This was a common complaint with people who ran GalCiv2 on Mac laptops and users usually found that using a cooling pad beneath the machine solved the trouble.

On the other hand, if it's self-rebooting, then it's probably a driver issue causing a Blue Screen, but windows is not set to display them. The most an app or game can cause on its own is a crash to desktop or a hang.