Videos are Stretched

When using Media Center to play videos, it is automatically stretching my
16x9 videos to fit the 4x3 TV I'm playing them on rather than keeping the
proper ratio and placing black bars at the top and bottom.

When I play a video DVD, it works fine. The playback is in the aspect ratio
that's on the DVD. I am copying my DVDs down to a media server for the
house. When I play the copies from the hard drive, that is when the problem
occurs.

Any ideas?

TIA!