You may need to just reconfigure X and start the config again. Use the terminal directly (ctrl+alt+f1 most likely), then login and run the following:
sudo /etc/init.d/gdm stop (or kdm stop if you are using kubuntu)
sudo cp /etc/X11/xorg.conf /etc/X11/xorg.conf.backup
sudo dpkg-reconfigure xserver-xorg
Choose either VESA or ATI driver - VESA will be the lowest common denominator. It should always work - and ATI will give you the default open source ATI driver.
It's pretty straightforward to reconfigure X - most likely one of the packages that you installed just conflicted with something.