If you have installed Win7 on an existing PC, and suddenly find yourself needing or wishing to
take advantage if the enhanced performance from ACHI (over legacy IDE)...
In regedit, go to HKLM:SYSTEM:CurrentControlSet:services\msahci
Click on Start, choose MODIFY, and set the value to zero.
Shutdown regedit, shutdown Windows, power cycle PC and go to your system BIOS setup
and enable your controllers AHCI support and save the config.
Boot into Windows, it will load an AHCI driver. Reboot again and the machine will be
running in AHCI mode.