What version of Ubuntu are you using Hardy Heron 8.04, or Intrepid Ibex 8.10? INtrepid is the newest. Also, what type of computer is this? Can you post the specs?
To reconfigure X for your video card and monitor, boot to recovery mode and login. Run: sudo dpkg-reconfigure xserver-xorg. Answer the questions as best you can. It is usually good to just accept the defaults that are given. When you reach the end of the reconfigure process, just reboot by typing reboot. When the system is back up, select normal login and see if it works.