Press Menu, Settings, Screen Adjustments, HDTV, and make sure 1080p/1080i or 720p is selected (pick the one that matches your TV) and the aspect ratio is set on 16x9.
When viewing a program, press the "FORMAT" button in the lower-left corner of the remote and make sure it's on "Normal".
When viewing a program, press the "FORMAT" button in the lower-left corner of the remote and make sure it's on "Normal".