If you installed any of the Ubuntu flavors (Ubuntu - default with the Gnome environment, Xubuntu - a flavor that uses the Xfce desktop environment, or Kubuntu - a flavor that uses the KDE desktop environment), you can switch between them without their respective install CD's by installing the right set of packages.

I recently ran into this because I used wubi to install Kubuntu, but found that KDE was a bit "busy" for my liking. (I have a short attention span, having more things on the desktop ended up being distracting for me.)

To install Ubuntu desktop:

sudo apt-get install ubuntu-desktop

To install Xubuntu desktop:

sudo apt-get install xubuntu-desktop

To install Kubuntu desktop:

sudo apt-get install kubuntu-desktop

If you're settled with switching from one environment to the other and would like to remove all the packages from the one that you no longer use (for example, xubuntu-desktop), do the following:

sudo apt-get remove xubuntu-desktop
sudo apt-get autoremove