When you have a thick black border around a TV picture, it's not the TV causing that, nor the cable box....
Usually it's the TV station that is using a 4x3 picture in a 16x9 frame. You've seen that on pictures hanging in your home -- a smaller picture surrounded by a white or cream matte.
TV stations can do the same thing with their picture.
A good example is what you are watching, a better example is ESPN with their ESPN side boards, but the one that I recommended you use to verify it's not a cable or TV issue is Discovery's High Definition Theater Channel.
It always fills the screen.
Try channel 1144 for HDT.
Or use your guide around that channel number.
Now if you don't have Hi Definition Cable let me know that, ok?
Also you can't get HD on anything but the HDMI and COMPONENT inputs on your TV set. Video 1 or AV 1 are low def inputs, and you'll only get a 4x3 picture unless you choose a widescreen Psize for that.