[Fingers]

If this is the demo or any version prior to 1.2 then the 50% CPU usage on a dual processor system is normal. The game simply uses up as much CPU as the operating system will allow, like most games do...

I strongly suspect that you're either somehow running the game with Microsoft's OpenGL implementation instead of the correct ATI driver, or the 3D accelerated user interface in Vista is using up your GPU power (if you're using that). Your hardware itself is slightly higher-spec than what the game was developed on so that's not the problem.

Personally, I'm not going to get a Vista system any time soon. Maybe after a couple of service packs it'll become usable.