Quit a few programs will do what you want, the one I like to use to trim the video whilst converting to a DVD is WinAVIVideoConverter.

Note to preserve the shape somewhat you need to trim the sides as well as the black bars off.

Generally I find either 10% off the top and bottom, and 5% off the sides for marginally letterboxed video is ok, with 20% off the top and bottom and 10% of the sides for badly leterboxed video works for me.