You need a program that does mp4 to mp3. You can use torrents also to download music.
Torrentspy.com - The Largest Bit Torrent Search Engine has mostly everything, I use bitcomet to torrent. I barely torrent though, I use Ruckus to download my music, it's free and legal if you have a .edu email adress. However all songs downloaded with Ruckus are encoded with a license so you can't put them on a music player, you need to download fair use to get rid of the DRM license. Might be too complicated for an average computer user, I say just go with Acou suggested and download torrents. Or, just search for mp4 to mp3 conversion in google and you can find a program that will convert your itunes songs so that they will work in windows media player, most likely it will be free also.