Well, to exit your X-Server, you have to press ctrl+alt+f1, then login as root, and enter 'init 3' to switch to runlevel 3. ('/etc/init.d/xdm stop' should also work).
Now you should be able to run the Nvidia installer.
"Computer games don't affect kids; I mean if Pac-Man affected us as kids, we'd all be running around in darkened rooms, munching magic pills and listening to repetitive electronic music."
-- Kristian Wilson, Nintendo, Inc, 1989.