Unable to start up demo

[eeerrrr]

just got the demo installed. when i try to get it running all i get is this error msg -
.\.BMRender\rmode.cpp(1583): DirectX call failed: -1, -1,D3DERR_INVALIDCALL (The method call is invalid. For example, a method's parameter may not be a valid pointer)

ive seen other ppl on the forums with this prob but no explanation on how to solve it. my specs are -

intel core 2 quad Q6700
4gb ram
nvidia geforce 8800 ultra
windows vista ultimate 64 bit

hopefully someone can shed some light on why i cant get this great looking game started. thanks