Root isn't supposed to log in graphically. It's a security risk and completely unnecessary. You should always log in as a regular user, then run root commands from a terminal using su or sudo. Most distros disable the ability to log in to a gui as root by default.
If you need to run a gui program as root, you can use gksu or gksudo (if running gnome), or kdesu (in kde), to launch the program. I'm sure there are other similar options also available.
You can also configure your x-server to allow root to directly launch gui programs. This has been covered many times, so please try a forum search if you want to set it up.