You can turn a USF into a Windows Media Player skin.
But as a practical matter, there's really only 2 GUI skinning formats today (and only 1 on Vista). WindowBlinds (XP/Vista) and XP msstyles. (Vista msstyles are just resource DLLs renamed, there's very little format to them).
When USF was designed, it was predicated on the assumption that there'd be several WindowBlinds competitors like there used to be (eFX, Illumination, Chroma, etc.). So the idea was that someone would make their skin as a USF and then export it into various formats.
USF doesn't support msstyles because Stardock couldn't get a license from Microsoft to make .msstyles (every method of modifying .msstyles files is probably illegal if you want to get right down to it).
