Stereo Mix Issues

Hello I just bought a Hp Pavilion all-in-One PC, and it has stereo mix on it. however when i try to use it nothing happens. I can plug up my mic to the mic jack and enable stereo mix, when I do this i can see the level bars moving on the mic when I talk but the stereo mix does nothing. I am running windows 7 with a Realtek High Definition. This is the same exact operating system and sound card that is in my Toshiba laptop and the stereo mix works on that. And I have updated the drivers on the desktop and nothing. Hp cant even help they are dumb and have no idea what i am talking about. Any ideas how to fix it on the Desktop?