Hiya, chances are you are booting into a display manager, and you are in runlevel 5 (disclaimer: I have never used Mandrake). You can try two things, as root:
# /etc/init.d/xdm stop
xdm might be called wdm, kdm, or gdm depending on setup. It's more than likely kdm or gdm. Or you can try:
# init 3
That will put you in runlevel 3 which, in Linux excepting Debian and some others, is multi-user with no display manager. This will kill X as well.
CTRL-ALT-BACKSPACE with a display manager will just bring you to the display manager login screen, with X still running. If you put it into runlevel three to install the driver, do "init 5" to start the display manager back up.
Hope this helps!
