Thanks Tony & BetaNews for the heads up here. Style XP allows you to easily add new visual styles or themes to Windows XP. It has no service or executable code, so there is no worry about your machine slowing. Instead, Style XP converts XML-based themes into Microsoft's format, which is natively read by Windows XP. It only supports themes constructed specifically for Style XP.
Download: Style XP Beta 4
News source: TGTSoft
At the time of posting their site is maxing out so be patient!