I responded about halfway to this question a long time ago. The good new is, I have done this. The bad news is, it took me ten or so tries and I did not keep notes. I'll tell you what I remember.
First of all, you stated that you installed XP recently. If you are willing to re-install, this may be simpler. Is your XP install disk a base version, or one that's updated with the service packs? I believe that it wasn't until SP2 that XP handled AHCI mode. You can slipstream SP3 with your install CD and build a new install CD that will work.
What I did started with an image backup of my boot drive, which I restored to a new drive and tested. This allowed my to go back to a working system every time I screwed it up. The trick is to download the current drivers, open Device Manager, identify your controller (if there are several, you have to select the root one for your chipset controller, but this should not be an issue with a notebook) pick Update Driver, and choose the downloaded driver pack.
Then comes the hard part. You have to pick the correct option for your controller as an SATA controller. This took me several tries. What happens is that it replaces the driver for the system drive. Then, when you reboot, the boot will fail. But if you change to AHCI mode before rebooting, and you picked the correct driver, it will boot! If you got it wrong, reset to IDE mode, restore your backup, and try again.
There may be a simpler solution, but I have not tried this one. Using an XP distribution disk slipstreamed to SP2 or SP3, do a Repair Install.
Let me know if you try any of these methods, and if it works or just wastes your time. Good luck.