Without more info, we won't really be able to help. What version of Windows 7 (i'm assuming 7 since you posted in the 7 section on the forum)? By this i mean, Beta, RC, or RTM? What video card are you using? Are the drivers you're using for it WHQL certified, and completely up to date?
Hmm... i played with the Windows 7 RC version for awhile on my main system with a GTX 295 and didn't have any problems like you are describing. Perhaps you should consider what area51 said and reinstall the video driver just in case? Latest Nvidia driver for Windows 7 should be version 190.62. Make sure that's the one you downloaded and also make sure you remove the existing driver first.