Ideally you should get any system drivers from the PC maker's website as sometimes their drivers are the only ones that will work. Since you haven't disclosed the make/model, I'm using the motherboard details instead. But as I say, drivers from PC maker is the usual route (unless you built it yourself of course, or it's an obscure brand).
Your motherboard is ASUSTeK P5P800
Go here:
http://support.asus.com/download/dow...Language=en-us
Ignore the left-hand column.
Where it says "Please select the product" there are 3 drop-down selectors.
Select in this order from the top:
1. "Motherboard" (allow page to refresh)-->
2. "Socket 775" (allow page to refresh)-->
3. "P5P800"
Click "Search"
Drivers for your board should appear on a new page.
You need the audio section so click the + sign against 'Audio'.
Choose the driver for your operating system.