Acer laptop " audio device missing"!

Hello,
My friends was on my computer deleting files and things because he thought it would make my computer run faster. He must have deleted something important because now my Acer laptops (with windows XP) sound is not working at all. I went to the Device manager and the audio device on high definition audio bus has a yellow exclamation next to it. It says that it is not installed so I try to reinstall it then it says it was not installed because the wizard cannot find the necessary hardware. Also the little volume icon in the bottom right of the desktop is missing. Please let me know what I need to do to fix this.