Changing the default Desktop Environment

If you usually start your X server by writing $ startx (thus you don't use a graphical login like GDM or KDM) you need to edit the .xinitrc file placed in the home directory. Create immediately it if you don't have :)
$ touch ~/.xinitrc
$ emacs ~/.xinitrc

write the name of the DE or WM you wish to start, here it's an example of the content
#!/bin/bash

#PROGRAMS TO AUTOMATICALLY START
gkrellm&
gaim&
#gdesklets start&
#kopete&
#sim&

#DE o WM da avviare
startkde
#gnome-session
#exec fluxbox
#exec fvwm2

if you want to switch from kde to gnome just comment startkde and ucomment gnome-session.