When switching between ATI and Nvidia drivers, lots of stuff gets left behind.
Here is a procedure to insure that you get the best possible install:
1) Download driversweeper to your desktop, or wherever you can find it easily.http://downloads.guru3d.com/Guru3D---Driver-Sweeper-(Setup)-download-1655.html
2) Download your new driver to the same place.
3) Add/remove programs, uninstall any Nvidia or ATI vga drivers and programs.
4) Boot into safe mode. Press F8 repeatedly while booting to enter safe mode. Your display resolution may be minimal.
5) Run driversweeper. Check the selection boxes for the removal of all nvidia vga drivers and ati drivers.
6) Rerun driversweeper. All such components should be gone.
7) Install the new driver from the desktop or saved location.