NVIDIA did just release a new set of drivers last night (9/13) for
version 306.23 which they say is WHQL* for Vista, Win7, and Win8.
If you are still having trouble, you could try reverting to
the last 64-bit WHQL version (301.42), which was released in May2012, though NVIDIA is strongly recommending everyone use the 306.23 version (for whatever reason, probably the Win8 support).
Please note that all the drivers I link to above are the 64-bit versions for Win7/8/Vista. If you are running 32-bit versions of XP/Vista/Win7, you will need to choose the 32-bit version of the drivers instead.
Note: According to NVIDIA, v306.xx will be the
LAST version of their GPU drivers which will support the older GeForce 6xxx and 7xxx series (6800 Ultra, 7900 GS, etc), so if you want a version that works with your older (possibly AGP) graphics card, you might want to grab a copy of whatever 306.xx version (this one or a later one) ends up being the last release to keep around juuuuust in case.
--Patrick
*This means that Microsoft has certified it as A-OK with their OS, if you didn't already know what that means.