Did you uninstall your ATI drivers before plugging you new nvidia card? If not, download this : http://www2.ati.com/drivers/cat-uninstaller.exe
It will remove every trace of your ATI drivers. Reboot and if you still have the same problem, uninstall you nvidia drivers via Add/Remove programs, then reboot and don't let windows install its own drivers. Cancel it and reinstall the latest ones.
Good luck.