For some reason, you didn't configure X correctly during the install. You need to rerun the Xconfigurator by running (as root)
That should launch a text based "wizard" that will try and re-setup your Xserver. Make sure that you know the brand and model of your monitor, video card, and general info like how many buttons are on your mouse. That will regenerate a replacement for your /etc/X11/XF86Config file. My guess is that right now, if you look at that file, either the Screens subsection is blank or that there is no reference to the Screen under the ServerLayout Section.