The Realtek drivers can have a problem with WinXP SP3, giving an error code -5009 when you try to install.
To get round this you need to do the following:-
Remember to unzip the archive that you downloaded!!
Ensure that there is no current sound driver installed by using The Device Manager to UNINSTALL any Realtek drivers
Use "Rescan for new hardware" and say that you know where the files are. (Give the path to your directory with the latest setup.ini in it.)
My path was C:downloadsRealtekHDAudioWDM_R227
Once I did this, It installed fine and device manager showed ATI Function Driver for High Definition Audio - ATI 791A and Realtek High Definition Audio. Remember to restart your computer to get everything updated properly.