Saved .avi not same as .wmv

When I save my project as DV-AVI (so I can burn it to a DVD) Movie Maker adds
a clip into the resulting .avi! This doesn't happen when I save the file as a
..wmv. The .wmv file is 6:44 long, and the .avi is 9:37 long. Has anyone seen
this before? Any idea how I can fix this? It is so frustrating!
Thanks,
Melanie