The first thing you are going to have to look to is altering the length of the song itself. The likelihood is that you only want part of the song in your Windows Movie Maker project and only at a certain part of the video. Unlike dragging videos into the Timeline, when you drag an outside audio track into the Timeline it will go exactly where you put it and not readjust to the very beginning of the Timeline display. From here you can reposition the audio track according to the timeframe set up by the video clip arrangement you have established.
Then you can begin to alter the audio track in the same way you would to video clips. You can drag in toward the center of the clip by clicking on the beginning or end and moving in that direction. This is how you shorten the clip either from the beginning or end. The best way to do this is to actually remove both the beginning and end, unless you want a dramatic moment in the music as when shifting energy in the clips.