Is it a widescreen TV? If so, I'll assume the picture is simply displayed in the wrong aspect. All widescreen TV's have an aspect button and options to view in various display sizes.
Look on your remote for a button with a sizing symbol on it. (ie. probably arrows or similar). Each time you press it the picture should cycle through each picture aspect. eg. 16:9, 14:9, 4:3, then some others maybe named Wide, Cinema or Zoom. If there is an aspect named Auto or Wide that should do fine, otherwise set it to 16:9.
(If you've already tried that or this is not a widescreen TV then overscanning would possibly be due to corruption of the EAROM, low EHT or other regulation problems.)
Cobweb
July 2006