Need way more information. What video card? What monitor? What X11 driver did the installation install?
To get a low rez display going, try this:
Make a backup of /etc/X11/xorg.conf first.
Edit /etc/X11/xorg.conf using pico editor or vi or whatever.
Find a section that looks something like:
Identifier “ATI Technologies, Inc. Radeon Mobility 7500 (M7 LW)”
Option “DynamicClocks” “on”
Edit the “Driver” element to say “vga”. This should get you a low rez X-Windows display. If it works, then make a backup of that version of the /etc/X11/xorg.conf file. Now go into your desktop as root, and re-configure your X11 display for your video card and monitor, you should be able to get a combination of settings that will give you a decent resolution and color depth.