I described my very similar problem in the thread that Radnor linked. My user account accounts boot to a login box, where all the login accounts are listed. If I select one of the user accounts, the X window goes away, then is reloaded, then the "Login at Box", and finally the background returns. This will repeat ad nausium. However, if I select Root, then the KDE Desktop Environment is loaded. I have tried going to runlevel 2 (Ctrl-Alt-2) and entering startx. It doesn't work as a user, but does as root.