1.First make Vista backup. How to do it all informations here.
2.Update Your Bios here.
This is very important for this model. If You will not do this Your audio may not work in XP.
Now You need to slipstream Your Sata drivers (only ICH9M) with nLite.
Download Nlite: Download
Download sata Drivers: Download
Read the Guide here: Slipstream Guide
Be sure also that You are using WinXP SP2 version. If you haven't got SP2 version here you can download this service pack: Download