Make sure that the computer does not boot into XWindows automatically... if necessary, remove "gdm" or "kdm", or remove symbolic links pointing to them in runlevel dirs in /etc/rcX.d. You can check the default runlevel from /etc/inittab.
Then edit the XF86Config-4 (or xorg.conf) in /etc/X11 and remove all fglrx stuff. You should use driver "radeon", not "fglrx". It might be best to create or restore a previous XF86Config-4. Editing a config created by the ATI installer is probably not the easiest way out.
Also, if you did not already do it, remove all ATI kernel modules. The installer should have a commandline switch to do an uninstall.