I take it your new PTV is 4x3 (not widescreen). You need to set the DVD player to 4x3 or 16x9 depending on the TV you have, not the DVD you're playing. If you have a widescreen TV, or your TV is able to "squeeze" an anamorphic image, select 16x9. Otherwise select 4x3 for regular 4x3 TV. This will only affect DVDs that were prepared for displaying in enhanced 16x9 mode. OK, here's why.
Movies today are shot originally in some widescreen format, anywhere from 1.78 to 2.1 and more. This specifies how much wider the picture is than the height. A regular TV is 1.33 times wider than its height.
If you want to see the whole movie frame they shot, you need to fit the width of the original image inside the width of your TV screen. Because the ratio of width to height of your TV is smaller (the TV has a relatively higher image), when you place the whole width of the movie image in it, the movie image is not as high. Hence the black bars at the top and bottom.
To provide an image that uses the whole TV screen, the DVD producers actually cut out part of the width of the original image so the width to height ratio is now the same as the 4x3 TV. They do so with a technique called Pan & Scan, where they select an almost square section of the original screen by "panning" around the wider original image left and right. So when you see two people talking in Pan & Scan, there may actually have been three people in the original scene.
Not what I want, but if it's more important to get rid of the black bars than to see the original movie as it was shot, so be it :-).
A good similar situation is printing the negatives from a 35mm camera on 8x10 inch paper - they usually chop some of the width off. But on 5x7 inch paper you get most of the negative image. It's all in the ratio of width to height of the original image, vs, the ratio of the display medium.
Check around - I think DigitalBits.com has some real info and much better explanations on how all this works.
