Per Ramhound's comment, assuming Windows 7:
Control Panel > System > (at top left) Device Manager.
Expand "Display Adapters" by clicking the + next to it.
Selet the nVidia by clicking on it once.
Right-click and choose Properties.
Go to the Driver tab. Cick on "Rollback Driver". That should revert you to the last one you had before you did the download.
It is also possible that you installed a driver that isn't matched to your video card. In that case, you may be getting a default display driver. You'd know that by going to the same location, and if you didn't see the nVidia display adapter listed, then you may be on a generic driver. In that case, you can go to your system mfr's site and download the display driver for your system, or you might have a support CD/DVD that came with the system and included the drivers. Many systems have all the drivers right on the C: drive; i.e. Dell usually puts theirs under C:\DELL.