I think this is what you are looking for. robN8R (who I think is on here as well) posted it on the zd7000 forums
If you want the system to stretch your game to fit the widescreen "Use Display Adapter Scaling" under DisplayProperties/Settings/Advanced/GeForceFXGo5600/FlatPanelDisplay. If you don't want the system to stretch the game select "Use Centered Output". If you want the aspect ratio to remain fixed and have the system scale up to fit as much as possible on the screen, chose "Use Fixed Aspect Ratio Scaling".
I have seen a few complaints about the widescreen stretching. This should help resolve this. Although I like the widescreen stretch, I'm afraid it might ruin my game when I switch back to a normal monitor. Not that I had such a great game to begin with. hehe
Even if we don't come up with the solutions ourselves, we scour the net looking for them. :D
Hopefully you have a GeForce-based chipset, so this will work for you. If not, you might now be able to contact ATI asking about "Fixed Aspect Ratio Scaling" and have them know what you're talking about.