It happened to me once with nautilus (the konqueror of gnome, it's far away from it btw, very poor one). When nautilus starts up it replaces your current kdesktop with the gnome one. And then you exit kde and enter again it's still there because kde remembers the apps that you left ran the previous time and runs them again.
If this is the situation with you (if you have once started nautulus) here is the solution:
1. start one konsole
2. execute 'killall nautilus' command - it will kill all instances of nautilus
3. exectute 'kdesktop' command it will the desktop of kde and it will replace the ugly gnome desktop.
4. next time when you login your kde should be normal
5. never start nautilus again
6. same as 5
7. too, and so on
here is a paste of my konsole:
anton@anton:~> killall nautilus
anton@anton:~> kdesktop
mcop warning: user defined signal handler found for SIG_PIPE, overriding
anton@anton:~>
good luck