These instructions are for Ubuntu 12.04 running the Unity desktop on which the xfce4 window manager was installed using either the Ubuntu Software Center or sudo apt-get on the command line.

The instructions may be the same for other versions of Ubuntu, but no guarantee is made.

When the Xfce window manager is installed in Ubuntu 12.04, it installs several packages and libraries it requires as well as some optional packages. Not all these packages and libraries are uninstalled when you uninstall Xfce. Several packages (like thunar) do not get uninstalled.

Quick Uninstall

This is not recommended because it will remove all packages normally installed with Xfce. This may include packages you previously installed (like thunar) or were installed as dependencies for other programs.

Note: this uninstall command assumes Xfce was installed on a fresh Ubuntu 12.04.

1) Open a terminal (instructions can be found here).

2) Enter the following command (it is very long and a single line):

sudo apt-get purge --auto-remove desktop-base exo-utils gtk2-engines-xfce libexo-1-0 libexo-common libexo-helpers  libgarcon-1-0 libgarcon-common libglade2-0 libjpeg-progs libjpeg-turbo-progs  libkeybinder0 libthunarx-2-0 libtumbler-1-0 liburi-perl libxfce4ui-1-0  libxfce4util-bin libxfce4util-common libxfce4util4 libxfconf-0-2 orage  tango-icon-theme thunar thunar-data thunar-volman tumbler tumbler-common  xfce-keyboard-shortcuts xfce4 xfce4-appfinder xfce4-mixer xfce4-notifyd  xfce4-panel xfce4-session xfce4-settings xfce4-utils xfce4-volumed xfconf  xfdesktop4 xfdesktop4-data xfwm4 xscreensaver xscreensaver-data

It is easiest to copy the line and paste it into the terminal (right-click on the terminal and select Paste from the popup menu).

Preferred Uninstall

The better way to uninstall it is to follow the directions on this page.

This will show you on how to uninstall only the packages that were installed when Xfce was installed. The instructions are generic and work for any installed package.

It involves:

identifying which packages were installed and
creating a command to uninstall only those packages

It leaves intact any packages you may have installed (like thunar) or that were installed as dependencies for other programs instead of blindly assuming that they were installed along with Xfce.