apsivam, Slackware uses runlevel 4 for XDM/GDM/KDM/*DM. Most other distros use runlevel 5.
nemesi, what's happening is that X is not allowing connections from different users (root) to your user's X session. This is good for security, but bad for convenience, as you can see. Some will tell you that it's bad to run X apps as root, which is true to a point, but there are sometimes wher it's convenient or necessary. In order to do so you can try a couple of things...
As user:
Which will allow all local users to connect to your X session.
Or, as root, this may work:
Try each.
--Shade