If I play a DVD on my HDTV I get a thin black horizontal line in the picture a few pixels from the top. Connecting my computer you see the line on the top bottom and sides of any video or DVD. Why?

On your TV, you have the ability to change the aspect ratio of the picture. What's happening is that you may have the aspect ratio improperly set so that the DVD's picture doesn't fill the screen, or possibly the video was filmed in an aspect ratio that's different that our current standards (maybe something originally ilmed in a different video format).

On your computer, it sounds like you're seeing the edges of the viewing area of your monitor. ALL CRT (tube type monitors) have a usable viewing area, but also conatin a black line around the top, bottom and sides.