How to change desktop environment kde to xfce4

Hi,
I am tring to remaster the version of Knoppix 5.1,and I want to change the default Desktop environment kde to xfce4.Because Xfce Desktop Environment is a lightweight desktop environment and everything goes faster.The fellow is what I have changed.At chroot environment,I modified the file /etc/init.d/knoppix-autoconfit,and I uncommented the line 495 and change the DESTTOP="kde" to DESTTOP="xfce4" .
line 495 case"$DESTTOP"in gnome|enlightment|kde|larswm|xfce|xfce4|lg3d|ratpo son|blackbox|openbox|windowsmaker|wmaker|icewm|flu box|twn|nx|rdp|kiosk);;*) DESTTOP="xfce4"
line 496 [ -n "$DESKTOP" ] || DESKTOP="xfce4"
,and I alse modified the file /etc/X11/Xsession.d/45xsession,and changed the line 77 [ -z "$DESKTOP" ] && DESKTOP='kde' to [ -z "$DESKTOP" ] && DESKTOP='xfce4',and maked Live CD iso.After that ,I tested the iso in the WMware Workstation,but encounter ed the error:
Probing /Loading AGP modules ...
Starting the X Windows System...
Unloading Unneeded AGP modules
X Windows session terminated with errors.
Please check and retry '/etc/init.d/xession start'

Thank you very much for help![/img]