I'm no expert, but a similar thing has happened to me (with Gnome on Solaris, so OK not that similar
) - you log in but it takes you straight back to the login screen.
In my case, the problem was a bad config file for gnome.
In your situation, what I'd do is log in in non-graphical mode (ctrl-alt 1 to get a console login), and move my .kde (or is it .kderc, I forget) directory to somewhere different. Then KDE will recreate all your config files.
If that doesn't work, move everything in your user's home directory to some other directory (to get absolutely every config file) and try the same. Or you could create a new user.
If you need help with any of the things I've described above, post back.
--Tim