If anyone is looking for a free and legal way to get music for trailers, and don't mind it being a random song that almost no one has heard before, a lot of indie music sites offer free downloads of songs, which are completely legal since the artists uploaded them there themselves. Some good ones are MP3.com and ArtistDirect.com.
Any .mp3 file you have on your computer, you can upload to WMV. I'm not allowed to tell you sites where you can illegally download, but they're around.