But there is an updated pack called Ubuntu 3.0 Skin pack (from the same author) which makes your Window XP look just like Ubuntu 11.04 Natty ("Bill" hates it :P).
|What the.... ;-)|
Starting with Icons to themes (including wallpapers) and most importantly you really can't miss the application dock in Natty (Canonical don't like when you call it "dock", it "should" be called "application launcher"), and the skin pack even comes with a dedicated dock called Rocketdock!.
Although when I was reading some of the comments, someone had complained about it making XP non-boot-able (yikes), so the author advices to make sure you have installed the service-pack 3 for XP in that case.
So I'm pretty sure you want the link right now!, so click here and get it. Extract and install the pack and reboot. Get more information from this official deviantart page.
For Windows 7 users...
Not to worry friends there is a separate version for Windows 7 users as well (not for Vista though). Instructions are all the same, get the pack from here and if you want more information on installing, etc... then please visit this page.