Alan in GSO, make sure that the channel you are watching is HD and that you have the box set either for 720p or 1080i. Helps to know what cable set top box you have, so that others can help you out better.
Check the settings on the TV. My Panny is always set to Full, the box set for 480i as Stretch. Some boxes let the tv do the scaling but there are some sets that do not do a good job so it is better to just set the box at the highest resolution the tv can handle for HD.