thomasorley, I know of 4 ways to do this in total:
1. As I outlined, loading the AHCI driver from a floppy disk during the Win XP Install via the F6 route. You do NOT need the floppy drive permanently installed for this - it could be only temporarily connected for the Install, then removed after. BUT you do need a floppy disk controller port on the mobo. Note that I do not think this can be done with a floppy drive mounted in an external case and connected via USB - at the time of Install, Win XP has not yet loaded any USB drivers.
2. SOME mobos somehow can let you use a USB thumb drive as if it were a floppy disk drive - I'm not sure how they make one appear to be the other.
3. The process Emerald linked to above. What it does is have you Install Win XP onto a SATA drive for which the port is set to operate in IDE Emulation mode. AFTER the whole thing is working that way, then the procedure guides you through a careful process of changing the port mode to AHCI, including all the little tweaks necessary to make this work.
4. A process called Slipstreaming. In outline, you do these steps:
(a) You need use of a second machine for the preparation steps here. First, download or find on your mobo disk the driver(s) you need for your machine. You can put them on your HDD.
(b) You will need a CD burner and some software able to burn an .iso image file to a blank CD-R - something like Nero.
(c) Search the web for info on slipstreaming, and for the lists of files and software you'll need to do this. Start with this link:
http://www.zdnet.com/blog/hardware/creating-an-xp-sp3-slipstreamed-iso-file/1848
Download the resources you need.
(d) Once you put all the parts you need into a specified folder on your HDD, you run the software tool you got for the job - probably nlite. What it does is copy all your Win XP Install CD to a HDD folder, merge into that the additional files like custom drivers you need added, then create a complete image (as an .iso file) of a NEW customized Win XP Install CD.
(e) burn the resulting .iso file onto a blank CD.
(f) Use that new CD as your custom Win XP Install disk. It will operate like a normal Install except that it will also install properly all the extra driver(s) you added to it. SO, if you added the proper AHCI device driver for your mobo to the collection that nlite created, you can Install Win XP to a SATA HDD set up in AHCI mode, and boot and run from it afterwards. The process makes the AHCI driver a "built-in" driver just as the IDE device driver is already.
This slipstreaming method is also a way that you could make RAID array drivers part of the customized Install routine, for example, so that you could Install to, boot and run from a RAID array without having to use the F6 key and floppy diskette route.