Display Problem,Proprietary Driver disabled
 

I am running UBUNTU 8.10 with a Graphics Card: ATI Radeon HD3870.
All was well, until I stupidly disabled the Proprietary Driver for the
Graphics Card. No at boot-up I get a black Screen with an Error:
Out of Range, H.Frequency:12.0kHz V.Frequency:11.0Hz.
Could anyone please tell me how to get back into the System to enable
the Proprietary Driver?
I do have a System Rescue CD, but I don't know how to operate it. After booting from that CD I get to the Point where it says:
System Rescue CD: root@sysresccd/root %
And a blinking Cursor waiting for a Command.

well, at first i would try to boot the ubuntu live cd, open a terminal, mount the ubuntu partition and read your /etc/X11/xorg.conf, you'll probably find something wrong. if you want to access your actual system you can do the same and chroot into the actual system. from there you should be able to run dpkg-reconfigure -phigh xserver-xorg (not sure, you can find out the exact command by reading your /etc/X11/xorg.conf). hope it helps.