I have solved this on my old LC-37G4U Sharp Aquos TV.
There should be buttons on your remote labeled "16:9" "4:3" or something similar. Try messing with them.
Also try the "zoom" button, if there is one. "Zoom" trys to zoom in on the picture to make it fill up the screen, instead of letter-boxing the video.
Another option is to press "menu" and look for an option that adjusts the aspect ratio. There is also an option to look at the "zoom," which is also under the menus somewhere.