Especially for XP users, here are the 4 official Microsoft extra themes for Windows.
Advantage: you don't have to patch any system file. You can use them directly.
The installer contains the following theme:
Users of Windows XP Media Center Edition should know this one: it's the default theme used by this version of XP.
A black version of the Royale theme (see above).
A derivative Royale theme inspired by the Zune interface.
This theme is the default theme of Windows Embedded, a special and customizable XP version for integrators.