Just downloaded Apup 16 August XP SP3 update, went OK. Opened Autopatcher and it shows KB 973815 in black with a check in the box. So it obviously has not recognised that it is already installed (other installed updates do show up in blue and unchecked).
It's listed in the registry at HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Updates\Windows XP\SP4\KB973815 and also in ARP cache and uninstall locations
Edited by JRosenfeld, 16 August 2009 - 10:34 PM.