Use the zoom button on your DVD player if it has one, this will zoom into the centre and fill the screen, of course you will lose a large part of the edges of the screen but hey thats life.
When a movie is made it has extra viewable areas top / bottom that are removed to create widescreen, these areas are designed into the movie filming so you are not losing anything they intend you to see, however with the advent of widescreen movies showing on terrestrial fullscreen TVs they added the top and bottom back on which is why you sometimes see boom microphones etc.
Now if you then create a fullscreen version FROM the widescreen content that has already been cropped top and bottom you are in effect removing the parts that they intended you to see (either side of the centre)
There is no way to get fullscreen from widescreen without either squashing the picture up so everyone looks tall and skinny or cropping the edges and ruining the move.
I would not do this because you will inevitably end up getting a widescreen TV in the next few years as full screen is being fazed out completely worldwide.
This is why I suggest you use the DVD zoom button which does the exact same thing without actually butchering up the DVD content, if your DVD player won't zoom buy a new player for about Â£30 from a supermarket as they all have these functions these days.
Just my two penn'orth