Replace your C:\Windows\Resources\Themes folder with the Themes folder in this archive.

It includes:

- "Windows XP Style" aka Luna: 
	the default Windows XP theme, with Blue, Olive, and Silver variants.
- "Media Center style" aka Royale: 
	the default from Windows XP Media Center Edition.
- "Embedded Style": 
	the default from Windows Embedded 2009. It's basically Royale in Navy Blue. It's quite tasteful.
- "Royale" aka RoyaleNoir: 
	Includes two color schemes:
		Royale: Believed to be identical to Media Center Style
		Royale Noir: Black and Grey version of Royale, very classy.
- "Zune Theme": 
	The official XP theme for the Microsoft Zune. Similar to Royale Noir; dark brown with orange accents.

These themes are virtually guaranteed to "Just Work" without any hacks or registry edits or any other effort.
To my knowledge, these were all developed (but not necessarily officially released) by Microsoft,
so they are 100% compatible and should not cause any issues.