I have had my Media Player on WinXP SP2 decide to play all my video files zoomed. It's quite annoying. My DVDs fill the freaking screen. I never could figure out why it does it. It's the same for my 163 and my 851, so I doubt it's your player. Probably a graphic driver, or a software player issue.
There is a setting for size of playback. Right click you window when a movie is playing and check the options it giives there, you'll find it.