In Movie Maker, you could try to create in a paint program your black bars then drop them onto the timeline's Title Overlay line in Movie Maker. Your paint image would have to be the same size as your DV video, 720x576 (PAL) and you'd need to mess with the size of your "black bars"
The idea being that your image is overlaid on top of your video, resulting in a narrow crop of your film.
It works for me in Premiere Pro should work in Movie Maker...
Canon HV30 | RODE VideoMic | Adobe Premiere Pro CS4.