It shouldn't be too much of a hassle if you already have Ubuntu installed.
just type this in a terminal:

sudo aptitude install xubuntu-desktop

I don't think the download exceeds 100Mb. Now in the login screen you select
"Sessions" and select Xfce, and you should have xubuntu running.
Installing the "kubuntu-desktop" package is pretty much the same, but it
comes with a hoard of KDE apps and messes up everything. It's also a huge
download (about 650Mb).