I recently built a new PC for someone and installed Windows 7. Having got near to the end of the operating system and software install, I realised that I had left the SATA controller in plain old IDE mode. I wanted to use the Advanced Host Controller Interface (AHCI) specification but was wary in case Windows wouldn't boot after the change. Fortunately (after a quick Google), I discovered it is a trivial thing to modify in Windows 7. Just enable the driver in the registry before making the BIOS alteration. All I had to do was change the value of the Start DWORD in the subkey below to 0, reboot and change the controller mode in the BIOS.
HKEY_LOCAL_MACHINESystemCurrentControlSetServicesMsahci
For detailed instructions, see Microsoft KB Article 922976.
You can check the controller is in AHCI mode through Device Manager:
AHCI Enabled
