Hey, you might need to reconfigure your X server. The command to do that in Ubuntu is :
sudo dpkg-reconfigure xfree86-server
Apart from this I cannot tell you more because I myself have never used Ubuntu.
Do this :
1) This site has a Ubuntu forum
Go to the forum and search for your problem there. It's most likely that somebody has already posted a similar problem.
2) Take a look at Ubuntu Warty Warthog 4.10 : Installation Help
or Ubuntu Hoary Hedgehog 5.04 : Installation Help
You'll find a lot of threads there, dealing with problems with X
Hope this helps.