If they already have XP, either:
(A) despite coming with Vista, the machines are too old for AHCI, or
(B) someone has already gotten the AHCI driver into XP, or
(C) someone has already disabled AHCI

(Which one might be different between the laptops.)

If you're looking at copying the image over from the old laptop to the new one, you're safe: even in case "B" the old laptop has the right driver.

If you're looking at a fresh reinstall, and you installed XP, following whatever procedure you used to do the XP install should work.

If weren't the one who installed XP, (B) is the one case that might trip you up on reinstalling it. It's pretty easy to recognize if it happens, as the XP installer will get partway in and say you have no hard drive. Fixing it requires doing an F6-driver load or slipstreaming the driver onto the CD, and if that doesn't immediately make sense to you, you probably don't want to do it.

If you have a Vista DVD and they came with Vista, unlike XP, my recollection is that Vista works with AHCI out of the box, and upgrading the machines might be worthwhile (and the memory; if they're on 2GB, Vista really does better on 4GB.)