I wrote a few months ago about skinning WinXP with custom Visual Styles by patching UXTheme.dll. Since then, Belchfire.net reports that a friendlier patching program called Multi-Patcher does what PatchXP does and more. It supports SP1 and non-SP1 systems, and even SP2 (beta version 2082) and Windows Server 2003. Perhaps more importantly, it allows you to easily reverse the patch by simply running Multi-Patcher again - nice safety net.

If you're looking to skin Windows XP with custom-made themes and Visual Styles available at sites like XPThemes.com and xptheme.info, this is something you should get.

Update: thanks to miahz for pointing out that I forgot to link to the Multi-Patcher article.