Can't create D3D Device

I have a problem with 3DGS.
I can't run the the engine, it says "Can't create D3D Device".
But why?

My System:
Athlon 64 X2 4800+
Geforce 7800GTX 512MB
1024MB Ram

I have installed the latest Nvidia Drivers and DirectX.
I use 3DGS Commercial 6.40.5
I have allready reinstalled 3DGS, and the Demos in the downloadsection: Car Demo,Bronx Street Walkthrough shows the same error.

Please Help!

restart you computer, it used to do it to me too, it screws up dx somehow so restart your computer and it should.