if you've run Demigod before (ala. you've just patched it), then search for a file called game.prefs and make it migrate to the Recycle Bin.
if, however, you've not run Demigod before, then you'll have to list your;
- graphics chip,
- specific driver version,
- current flavour of Windows used.
that should give us enough information to tell you what to fix, or point you in the right direction.