If you've updated to Nvidia drivers recently, you may have noticed a decrease in framerate by quite a lot, this is caused by two things:
The 3D Vision Stereo Drivers
The new "Ambient Occlusion Effect"
First of all, we're going to go over disabling the new "Ambient Occlusion Effect" as this is the largest increase in framerate.
Disabling Ambient Occlusion
1 ) Go to your desktop, right click on your mouse and select Nvidia Control Panel

2 ) Select the Advanced View option if you haven't already. 
3 ) Go to Manage 3D Settings and disable Ambient Occlusion, Now click Apply. and close down the Nvidia Control Panel.

Disabling the Nvidia 3D Vision Stereo Drivers
1 )
XP: Go to Control Panel and select Add or Remove Programs
Vista/Windows 7: Go to Control Panel and select the Uninstall a Program option(image below):

2 )
Select the Nvidia 3D Vision Stereo Drivers and click Remove or Uninstall and Change on Windows 7/Vista(can also be done by Right Clicking and selecting Uninstall and Change)
3. After uninstalling the 3D Drivers, restart your computer!
Hopefully this will sort out all of the problems to do with the new Nvidia drivers. Join me in my topic next week as I discuss configuring your World of Warcraft Graphics for the best performance without hindering quality!