I've run into similar problems in the past using the BT4 LiveCD on various desktops/laptops. There are a few posts on this forum addressing this problem (as well as other Linux distro support sites). You can try and reconfigure X by entering the following:
root@bt:~# dpkg-reconfigure xserver-xorg
You will be asked a series of different questions about your display. Try and manage the best you can or select the defaults where applicable. If things don't work initially go back and try a new setting or look up your display device for the specs rather than guessing.
I haven't used it yet but several other members make reference to using the following command :
I went and changed the xorg.conf file manually which I'm guessing probably did the same thing as using the fix-vesa command?
Anyhow, if neither of the above work, google is your friend. Good luck!