What is your nVidia driver version?
Right click in any open area of the Windows desktop
Select NVIDIA Control Panel from the popup window
Select SYSTEM INFORMATION at the lower left corner
Driver version should be the 1st line in Details
I have a GTX285 using driver 296.10
Many people had to update their nVidia driver to go from CS5 to CS6... so it would not surprise me if a Win7 to Win8 change also requires a driver update