I don't know if WMP can do that. I use VLC Media Player - it's an open source media player and has aspect ratio controls.
http://www.videolan.org/vlc/Edit: I don't really like the video board feed...they far too often forget to switch to the play from the player bio graphics, replay, ads, etc.
[This message has been edited by EnviroAg96 (edited 2/24/2010 3:12p).]