SetParent a FullScreen Game

Hi,
I need to resize a FullScreen Game (Dx or OGL).

Exemple:

800x600 FullScreen Game (at a monitor 1024x786) and put it in a 800x600 Window.

i'm trying to use SetParent and SetWindowPos.

If i Can SetParent the Game in FullScreen ( and don´t show my form), It can help so much.

Thanks.