# Install the gnome-fallback-session and Xfce
sudo apt-get install gnome-session-fallback xfce4 -y

# Check if the new sessions have been actually installed
ls -ail /usr/share/xsessions
total 48
drwxr-xr-x   2 root root  4096 2012-04-15 11:55 ./
drwxr-xr-x 307 root root 12288 2012-04-15 11:55 ../
-rw-r--r--   1 root root   233 2011-12-04 22:15 gnome-classic.desktop
-rw-r--r--   1 root root   205 2011-12-04 22:15 gnome.desktop
-rw-r--r--   1 root root   277 2011-12-04 22:15 gnome-fallback.desktop
-rw-r--r--   1 root root   188 2011-12-04 22:15 gnome-shell.desktop
-rw-r--r--   1 root root   208 2011-12-04 22:15 ubuntu-2d.desktop
-rw-r--r--   1 root root   185 2011-12-04 22:15 ubuntu.desktop
-rw-r--r--   1 root root  5808 2011-09-23 13:21 xfce.desktop

# select Xfce as default GUI
sudo /usr/lib/lightdm/lightdm-set-defaults -s xfce

# logout
/usr/bin/gnome-session-quit --no-prompt

Next Steps

– select in the login box “xfce session”

– login

– reboot after login to permanently so that this preference is permanently save

References

1. Make Ubuntu look and feel like Gnome:
http://www.omgubuntu.co.uk/2011/12/how-to-make-ubuntu-11-10-look-and-feel-like-gnome-2/

2. How To Change The Default Session:
http://askubuntu.com/questions/62833/how-do-i-change-the-default-session-for-when-using-auto-logins/62836#62836