Try this, not sure if i've got the exact driver for your model but its for a P5Q board. Worth a try.
Download the driver utility here: http://dlsvr02.asus.com/pub/ASUS/misc/u ... 321537.zip
Save the file in "Downloads".
Extract the ACPI_104321537.zip file inside the Download folder (do not change directory).
Next, click on Start and type in Device Manager
In device manager, locate the "Unknown Device" right-click on it and choose Update Driver Software
Click on the Browse my computer for driver software
In the input box, put in c:\users\USERNAME\Downloads\ACPI\ACPI64
with your actual username
Click next to see if it installs driver. Hope this works.
Check to make sure you have unextracted the contents of the zip file to Downloads... if you accidently saved it in say My Documents then the path to your driver folder would be c:\users\USERNAME\Documents\ACPI\ACPI64 ... just a heads up.