I'm basically a GNOME person when it comes to pick the desktop environment in linux os. I've been using GNOME for years with Fedora. Recently I wanted to try the KDE in my Fedora 9. So, I installed KDE from my YumEx which installed all the default and required programs for it.

But when I logged out and wanted to log in with KDE as desktop session, I got following error - "Could not start kstartupconfig4. Check your installation."
with only option "okey", which takes you back to the login screen. Re-trying same thing for several time resulted to regenerate same error.

So, with no hope, I started googling and did not find solution for a while. Finally I got into the one of the threads of fedora forum and followed a simple instruction to solve this. The steps are as follows...

Solution
After you install KDE, you have .kde folder inside your home directory [/home/yourusername/.kde]. Remove/Rename(recommended) that to something else.

 [hossain@localhost ~]$ mv .kde/ kdebackup

Logout from your GNOME or other DE and try to re-login using KDE. You should be able to login with KDE. You can remove that 'kdebackup' folder, as there is already another .kde folder with correct files and configurations.

Enjoy !!!