I have two more ideas:

1) In '/etc/inittab' should be a line saying something like (I'm currently at a SuSE-Linux system):
# The default runlevel is defined here
id:5:initdefault:
Check if this is set to runlevel 5.

2) In your homedirectory should be a file named '.xinitrc' or '.xsession' (it depends on the distro which file is used). It should contain the following line:
exec startkde
But I'm not really sure if this will work. What does your /etc/X11/XF86Config-4 "Device" Section say?