Once it's set to IDE mode the controller presents a different hardware ID which the AHCI drivers will not match up to, so Windows will not update the drivers as such.
Do not perform the following unless you are sure you are installing the right driver for the hardware present - To install them you would need to force the drivers to install by specifying the folder they are located in and choosing the 'Have Disk' method and turning off the compatibility check. After which you would NEED to enable AHCI in the BIOS.
You can find guides online - just google windows xp ahci driver install, there are lots of people doing what you are trying. Personally I wouldn't and would slipstream the drivers instead.
To install Windows XP from scratch with the AHCI drivers you would need either a floppy disk with them on (and a floppy drive), or slipstream the drivers on to the Windows disc.