Possible for player to open media with a fixed aspect ratio?

I have a scree resolution of 1280 X 1024 i.e. 5:4. I need a custom aspect ratio of 5:4 in order to view video files at full sceen w/o any black edges. Tried to change screen resolution during full screen, but it slows down considerably. Remember movie settings works only on files I opened before. :?

Is there anyway to select 5:4 as the default aspect so that everytime I open a file, it auto starts with this ratio?

Thanks.