Check the file "/etc/inittab"; there you can see what is the default runlevel of your system; let's suppose it is "runlevel 4", then go to the "rc.d/rc.4" (somewhere inside "/etc/") and there you will see something like
# Tell the viewers what's going to happen...
echo "Starting up X11 session manager..."
# Try to use GNOME's gdm session manager:
if [ -x /usr/bin/gdm ]; then
exec /usr/bin/gdm -nodaemon
# Not there? OK, try to use KDE's kdm session manager:
if [ -x /opt/kde/bin/kdm ]; then
exec /opt/kde/bin/kdm -nodaemon
you only have to switch them over.