theres an app out there that fixes it most of the time. Search for nvidia win2k refresh fix, or something similar, you'll find it. Funny thing is, to *me* it seems like the desktop is still 60hz, the mouse is slightly jerky.... but in games my framerate is fine, maxes at the 85 my refresh should be. try that util and see if it works for you.
FYI for some of you, this problem only affects nvidia cards in win2k (as far as I know it's a Det3 thing)