The K Desktop Environment

9.3. How do I replace the standard Linux text login screen with the KDE login screen?

First, you need to change to the "xdm runlevel" (runlevel 5 on RedHat systems) by editing your /etc/inittab file. In the file, you should have a line saying:
 id:3:initdefault:

Change it to:
 id:5:initdefault:

Now at the end of the file, comment out the following line:
 x:5:respawn:/usr/bin/X11/xdm -nodaemon
and replace it with:
 x:5:respawn:/opt/kde/bin/kdm -nodaemon
(the location of kdm may differ on your systems, though)

For changes to take effect immediately, type "init 5" at the shell prompt. Please note that it is risky to initiate a graphical login without checking beforehand whether it works. If it fails to work, you'd be in for a hard time getting back....