HDTV Keeps Auto Adjusting

I have my xbox 360 hooked up to a samsung T240HD with a VGA cable, and every time I turn on my system or tv, or change input sources, my monitor auto adjusts and I get two black bars on the bottom of my screen instead of having them on the top and bottom, which is the way it should be. I keep having to go into my menu settings and reset it every single time I turn on the system or change input sources, and it's really getting on my nerves. Is there anything I can do to fix this?