Howtoforge posted a tutorial about setting up Kubuntu 11.10 as desktop replacement for Windows

This tutorial shows how you can set up a Kubuntu 11.10 desktop that is a full-fledged replacement for a Windows desktop, i.e. that has all the software that people need to do the things they do on their Windows desktops. The advantages are clear: you get a secure system without DRM restrictions that works even on old hardware, and the best thing is: all software comes free of charge. Kubuntu 11.10 is derived from Ubuntu 11.10 (Oneiric Ocelot) and uses the KDE desktop instead of the GNOME desktop.

  The Perfect Desktop - Kubuntu 11.10