Start the computer in run level 3.
During the boot-up options screen, add a 3.
(that's a lilo example).
Once it's in runlevel 3, it should not start gdm and you will be in a console.
Once gdm (actually this is X doing this) locks up the keyboard and mouse,
you onviously won't be able to press ALT-CTRL-F1 to switch to a console.
Sounds like X is incorrectly configured.
Hope this helps,