The first PC was a Core 2 system running XP. I forced the SATA controllers to update to
these drivers, rebooted, and set the BIOS to AHCI. After that, it booted into XP without any trouble.
The second one was an Ivy Bridge/Win7 laptop I did yesterday. I used a
Microsoft FixIt tool, but it looks like that KB page lists the registry key that you mentioned earlier. After doing that, I installed the latest INF update from Intel, but that's optional (it runs fine with the default MS AHCI driver).