When you load the movie to virtualdub apply 2 filters (video->filters)
1 'Rotate' to get the movie onto the side and
2. 'resize' to fill out areas to get the movie to a 4:3 ratio
(widthxheight) otherwize MM will stretch it.
For step 2: If you have a movie that is 640x480 you have flipped it - it
is now 480x640. To make it 4:3 you have to widen the width to 854 (or
4:3 times your new height (640)).
To do this my settings are:
New width 480 (this setting means the width of the movie itself)
New Height 640
Filter mode: fiddle with this to suit your movie (it can affect the end
quality but I can never remember which I use so I always play with it a
few times - I think I generally use nearest neighbor cause it is the
first one to come up)
Expand letterbox and frame image is ticked
Frame width 854
Frame height 640
Fill colour - I use black but it can be whatever suits
Press OK