To get the correct AR, you need to use the 'ZOOM' button (don't know why

).
Is it mainly DVD's or Blu-rays that are the wrong AR?
If it's Blu-ray's, just set it to 'Video-Full Screen' or 'Default'.
If it's DVD's, the reason the AR isn't quite right is cos video on DVD's ISN'T stored in it's native AR (there are flags on the DVD which tells the DVD player what AR to display. If you play the video back on wmp on your PC (which doesn't correct AR automatically), you'll see that the native DVD AR is wrong.
For DVD, you'll need to use a combination of manually forcing 4:3/16:9 settings on your TV & 'Zoom' button to get the AR right