Cannot get video to play in full screen mode

Running Windows 7 Pro fully patched.

Windows Media Center was working well. I don't know what changed but now all videos, movies, etc all display with black bars above and beneath the picture. I think this is called letterbox. I have tried every setting and I cannot change the display to remove the letterbox.

I have checked the Windows 7 display settings to be sure they are correct.

How do I troubleshoot this problem?

Problem solved. There is a small device which converts the output of the pc to a composite signal. This feeds an old TV; one which was made long before computers were available. I did not realize this small box could malfunction and change the computer video output so both the television and the computer monitor video would appear letterboxed. I rebooted the small device and now all is working well.