4:3 Movies Have Side Black Bars on 16:9 Monitor

[nghwae]

When I play a DVD of a TV movie which has a 4:3 aspect ratio on my 16:9 monitor I have black bars on the left and right sides of the screen. I also get side black bars when watching some other movies that aren't 4:3. These bars show in small windowed and full screen modes. I tried changing some of the video settings such as extending the image to 16:9 but if the aspect ratio is 4:3 this results in unnaturally looking stretching of the image and top/bottom traditional widescreen black bars. Is there a way to make sure there are no black bars on the sides whether the movie is 4:3 or not, on a 16:9 monitor?