Change the Look of Your UX (Windows XP) -
These two themes are from Microsoft (however hosted elsewhere). They are very good looking and will give your UX a nice new look. These themes change the look of your windows and taskbar, and are easy to install, and easy to reverse if you do not like them. Here are some screenshots -
Zune Theme -
Zune Theme Showing Menus -

Royale Theme -
Royale Theme Showing Menus -

So there you have it, both look pretty cool, and its nice changing from the old XP look.
I don't wanna hear any complaints from Vista people because they get a awesome theme already : P Simply download and install the themes, then right click on your desktop and go to the appearance tab, they will be available under the "Windows and Buttons" drop down list, or simply on the Themes tab.
Here are the download links -