Are you using KDE? I believe there is a "log off" menu entry in there somewhere that will allow you to log off the user, then you can log in to root from there. Heck, I think there is a "switch user" menu entry as well. I can't remember for certain as I'm not using KDE these days.
If you are using XFCE you just hit the "quit" entry and when the dialog window opens, you click "log out".
There are also command line options available to you if those things don't work. Good luck!