Hi, I did this for a client the other day he was having problems running the setup file. Go here and download the latest drivers (do not run anything else suggested on this page just the driver zip file around 30 mb)
Unzip and take note of the location (typically it will be C:\users\Your user name\documents\unzipped\ac97 vista xxxx)
Now go to start search and type devmgmt.msc press enter. Device manager will open. Navigate to "sound video and game controllers" expand the tree and right click on Realtek AC'97 Audio and select "update driver software" when prompted browse to the unzipped file, reboot and if you want the sound manager let it install at reboot.
Note:- this is for 32 bit systems because the driver is not verified it will not run on 64 bit Vista or Seven