It can be very touchy.
It may be related to the manufacturer's hardware choices.
My friend got a new Windows 7 laptop for his work.
He had a problem with a chip programming device (USB), so we spent 3 days (in 2011) trying to enable AHCI.
We tried multiple methods outlined on various websites.
We could never get it to work.
Luckily it turned out my friend didn't need XP.
The external device that wouldn't work, actually had a problem with USB3 and not Windows 7.
I used the "slipstream the drivers into an install disc"
method, when I reinstalled my operating systems last September.
I used nLite
to create my new XP install disc.
It worked perfectly.