Music for Windows Movie Maker?

I am trying to add music to a slide show I am doing on Windows Movie Maker. It comes up with "The file C:\Doents and Settings\Owner\My Doents\My Music\My Playlists\SpringFling.wav.wpl is not a supported file type, and it cannot be imported into Windows Movie Maker." Why is this so. I am getting really annoyed by it. PowerPoint is a lot easier than this piece of Crop.