Auto zooming very frustrating!

I have an HD20 box... which is a directtv HD receiver that is currently set to 16:9. (I have a Philips HD TV)

I want to have all my HD programming fill my screen, but when standard definition programming comes on, I want to have the black pillar boxes on each side.

The problem is, it always AUTO ZOOMS to fill the screen, no matter what the programming is!

Therefore, if the program is standard definition, it zooms to fill the 16:9 screen, and stretches the image.

I hate this.

Is there anyway that I can set the settings to have the pillar boxes for SD, and Full screen for HD?

Thanks!