If the picture doesn't fill the screen on an HD channel, then the show wasn't recorded in HD and isn't meant to fill the screen. You were probably on a standard def widescreen broadcast on an HD channel if the tech had bars on all 4 sides when setting it up. You need to have the aspect ratio set at "normal", not "zoom" or "stretch" (or your tv's equivalents).
Some people like to stretch the standard def programs, but if you don't change it back when you're on an HD channel, then your HD isn't going to look right. If standard def programs are filling the screen, then you are either zoomed or stretched which will degrade the HD look.