can't seem to convert without letterbox

[papillon1977]

Im using prism to convert my mp4 files to .avi using the 1920 width and 1080 height with widescreen 16:9 option yet when i view the file it comes up in my windows movie maker and the videopad editor showing boxed in on all sides even before i edit. After when i save the edited version i make sure the settings are again on 16:9 and at 1920x 1080 as well but my dvds come out letterboxed. im wondering if its prism that created the first problem as it looks letterboxed in its .avi final output state even before i edit using windows or videopad editor.