Through your Nvidia Control Panel.
If your using the latest version:
Open the panel
Select 3D SETTINGS
Under that select SET PHYSX CONFIGURATION
In the right panel now you should see a diagram of your physical connection to the GPU.
Under SELECT A PHYSX PROCESSOR there is a drop down arrow make sure your video card is selected and not the CPU. Do not allow it to select AUTO. On automatic the computer will sometimes revert to using the CPU to process video images and this is what you DO NOT want. Manually put it on your GeForce card.
You should also read this thread:
http://www.subsim.com/radioroom/showthread.php?t=128517
To get the best settings out of your Nvidia card for playing Silent Hunter 4.