The best and working description of how to enable AHCI on Windows XP for a Fujitsu-Siemens Esprimo P5720 desktop is located at
http://forums.lunarsoft.net/topic/2746-enable-ahci-on-intel-ich9-under-windows-xp/This is correct link to the driver
Intel Matrix Storage Manager 7.6.0.1011 Kit 11666