i play avis on full screen. winamp plays them back a bit choppy, windows media player plays them smoothly but i'm not sure how to change the aspect. the default's black bars aren't big enough.
i forgot to mention vlc media player. i tried that as well and couldn't it to use thick bars without cutting off some of the actual content. subtitles were only half visible. i'm on a 12.1" screen so maybe that's why i'm having issue but i'd still like to be able to correct this.
Neros Showtime, part of the Nero Burning Rom Suite...is a pretty decent player.. You could also try AVS media player...http://www.avs4you.com/AVS-Media-Player.aspx
I have a feeling if you play full screen it's going to go with whichever dimension fits the screen res(width or height is closest.) The only way to get the right aspect ratio is to let it simulate it by being in a window.