I'm not sure about the menus. Certain dvd menus appear letterboxed on my computer screen but are 4:3 on a tv, even a widescreen one. Or is there something else you know about the menus.
Menus (and movies, but it's rarely used) have a flag that can make them appear 4:3 if the DVD player that is set up as 4:3, cutting off the left and right. I assume your computer is proclaiming itself to be a 16:9 display, and whatever DVD player you're using is set up for 4:3.
DE