You could simply kill the login manager that your using to login. Close all applications that you are using then kill the login manger that your using, for example, as root type:
If your using KDE [kdm is it's login manager] it will kill xwindows and boot you to a console login manager. Login as root and install the NVidia drivers. Then run kdm to start your kde login session and log into your regular user account.
I think if your using gnome it will be killall gdm-binary
Hope it helps,