Knoppix (at least up until 3.6) included the lightweight XFCE desktop environment as an alternative to KDE. The only thing is, it was a rather old XFCE version, something like 3.8. XFCE4 is still small, but looks a lot better than 3.8. I know that the current version released today is rather big and probably not stable, but what about 4.0.6? It's lightweight, and looks very "clean".