1. make sure, you do not have any package named "nvidia*" installed.
2. first log out or your graphical environment
3. switch to a virtual terminal ctrl+alt+F1
sudo /etc/init.d/gdm stop
sudo /etc/init.d/kdm stop
(depends on what you are running)
5. run the installer.
sudo /etc/init.d/gdm start
(or the same for kdm)