Oh.. after so many tries..
I finally got the driver which has the "stereo mix"
vista drivers sucks...
I've downloaded the Sigmatel Audio driver for XP from Gateway website. My laptop is also from them.
Now I installed it and its working..
If you have gateway laptop with Sigmatel Audio driver and you want to have "stereo mix" then down the driver from the link.
SigmaTel Audio Driver for Win XP
Download it then just open it and install. You don't have to do anything else.