Have you tried MSFT Live Movie Maker ? It lets you edit the soundtrack (change it).
Use your start button; search for movie maker. If not there, then you'll need to download Windows Live Essentials. And I think you can just install one component (Movie Maker). If not, just install the whole suite of tools.
Then run Movie Maker.
Import your video.
And then select a new audio track.
Then you can edit the audio tracks.