Windows Vista forum: No audio output device is installed TRIED EVERYTHING

I bought a sony vaio 2 weeks ago. I worked perfectly untill 4 days ago, when the sound suddenly was gone. I got the famous "No audio output device is installed" message on the lower right corner, and there is no sound device in my device manager either. I mean, there is no "sound, video, or game controllers" roll-out category in my device manager.

I tried every possible solutions. I reinstalled the realtek high definition audio codec driver from my Vaio recovery cd, but there wasno change. I manually added hardware in device manager but then there was a yellow exclamation mark. I restored my windows to the time before the sound was gone, but sound was still gone.

I was so frustrated, and decided formatting the computer would be the last solution. I did so, and the sound was back,,,, for ONE DAY.
It worked for one day, and when I turned it on the next day the sound was gone again...

Considering the fack that formatting fixed the problem for a while, it's definately not a hardware problem.

WHAT SHOULD I DO??????

I can't format my computer every 2 days....

can someone plz help me?