that's not the issue
the issue is, he's watching 4:3 content on a 16:9 display
you can't make it fit on 16:9 without either stretching it, or cropping it.
hit the M key while in the MythTV mode. you can change aspect ratio there for what you are currently viewing...
for recorded videos, simply use the Aspect Ratio menu item from the orbiter's main menu.