Possible Duplicate:
How to revert to GNOME Classic?
Can a Gnome2 like interface be built upon the new Gnome3 code?
How do I make an Ubuntu 11.10 setup running XFCE, or maybe GNOME 3 classic mode look and behave exactly like the classic Ubuntu setup of Ubuntu 10.10?
xubuntu-desktop
over existing Ubuntu installation may break other environments. You can installxfce4
for a minimal XFCE desktop, to just try things out. Once you make up your mind, you can set desktop of your choice as default - see http://askubuntu.com/questions/62833/how-do-i-change-the-default-session-for-lightdm-when-using-auto-logins – jnv Oct 26 '11 at 19:03