Hellhammer's link works for Windows 7 and Vista. I tried to follow a similar guide in XP*, but I had only one of the three relevant registry keys, and it was already set. I found another guide that was complicated enough that I'm going to try a clean install if I need AHCI+XP.
* I'm slowly migrating from XP/HDD/ATA to 7/SSD/AHCI. It's a minor inconvenience to edit the BIOS settings whenever I switch my boot device.