It should be pretty obvious in the bios. If it isn't, that setting may be locked out. PC vendors sometimes use customised bios's to lock out settings they don't want people messing with in order to ensure the PC continues to run smoothly. That supposedly creates less support calls for them.
You can always use the F6 option during the XP install and add in the AHCI drivers if needed. This will be your only option if it's locked out in the bios.