While WINXP installation is initiated, you should press F6 key to install onboard RAID driver. At this moment, you should install both "NVIDIA RAID CLASS DRIVER" and "NVIDIA NForce Storage Controller" driver. Then, the OS installation program can recognize RAID disk partition properly later.
AHCI is a programming interface for SATA host controller. It provides new features, such as NCQ and hot plug, to increase performance. If you want to enable AHCI, please kindly set "SATAAHCI Mode" as AHCI in the BIOS. Then, install GIGABYTE SATA2 RAID driver in Windows XP/2000 O.S.