Aspect ratio

[PTTP]

All I did till now was converting different kinds of .AVIs to MPEG1 or MPEG2. Or at least I tried. ffmpgX got the aspect ratio wrong most of the time. In the last VCD I made I almost didn't notice until there was a perfectly round full moon in the picture. I compared with the source AVI and it is indeed very circular. On the VCD it looks more like an egg (meaning in this case: an egg on its side, the black bars are to thick). From my experience I know that many people don't notice that stuff or just don't care. And in most parts of the movie you won't notice the difference.
But I wish there was a way to tell the program manually what aspect ratio the film has and how much black bars are needed to be add. Or a way to control it visually. Maybe like in this program:

http://mm2d.sourceforge.net/man_transform.php

Any chance for a solution on this?

[major]

Adding to to-do list..

[PTTP]

A good answer that lets me hope