Do you use a graphical login manager? I'm assuming that you do... As root, type telinit 3 in a terminal window. This will change your run level from 5 (I think that's the level Fedora uses for GUI, have a look in /etc/inittab) to run level 3. In run level 3, your X server isn't running and you can install the nvidia drivers.
After you've installed the drivers, type telinit 5 to change back to GUI mode.