It's likely that your aspect ratio in the cable/sat box is off, or that you have a ZOOM feature enabled. You might try calling your cable/sat company to see if they can walk you through setting up a 16x9 aspect ratio.

Once that's completed, make sure that you don't have a zoom feature enabled in the television. There are two parts to this equation - the source and the television, so both have to be on the right setting to properly fill the screen to see the picture as it was transmitted.