OK, here's something else you can try:
Originally Posted by georgelappies
At the login screen, switch to a virtual terminal with Ctrl+Alt+F1. Log in as the original user, unlock your home directory when prompted.
Now give these commands
This will remove Kwin and Compiz settings. The first command will give you the location of kwinrc. Please replace this in the second command, instead of the part in red. Depending on whether you used KWin or Compiz, one or the other command may give you an error, which you can ignore (for now).
mv ~/.kde/kwinrc ~/kwinrc_backup
mv .compiz dot_compiz
Now, restart KDE
and attempt to log in again (Press Ctrl+Alt+F7..F10 if the login window does not appear).
sudo service kdm restart
Please post back with results / errors.