That's normal to get black bars on the left and right sides of the screen. The only time you'll get the full aspect ratio (16x9) screen is when you're receiving an HDTV broadcast. If you have Comcast there are two channels call INHD which broadcast all programing in 5.1 DD HDTV. You should also be able to receive ESPN-HD, which broadcast most of it's programming if full aspect ratio.

Good Luck
Joe