are you sure that you have x-window-system installed
if not try
apt-get install x-window-system
if it returns that the latest version is already installed then that is not your problem. did you install using all of the disc's?? if you remember all of the steps that you took on installation then you can compare what you did with this http://www.osnews.com/story.php?news_id=2016
and see if you missed a step on the config side.