It is best to set AHCI in BIOS and then install the OS. But you can install the OS first and then change a setting in Registry and then set AHCI in BIOS. Otherwise, Win 7 does not boot.
I prefer to set AHCI first in BIOS. Why mess with registry settings afterwards.
I shall try the registry method first, to avoid reinstallation of the operating system. Thanks for the information about AHCI/IDE. Do you know what kind of performance difference there could be?