For a debian based distribution, I *think* the way to do it is to swtich from kdm to gdm as the default display manager. (assuming you already have gnome installed - if you don't you may want to go through dselect to find and install it or try apt-get install gnome)
I got this tip from
http://www.debianhelp.org/ which is a good debian specific resource:
Try /etc/init.d/kdm stop; /etc/init.d/gdm start (as root) and if that seems to do it, you can change the stuff in /etc/init.d permanently.
