To change the desktop from the default KDE desktop, just click K->Knoppix->Utilities->Choose/Restart KNOPPIX Desktop, or at boot type the cheat code desktop=name_of_desktop. Even the default desktop has a fresh look–KDE itself has updated packages and uses the PlastiK theme by default. I like gnome bettter.

http://www.oreillynet.com/sysadmin/blog/2005/06/knoppix_40_dvd_like_a_kid_in_a.html