AHCI mode is chosen in the BIOS before you install windows for the first time. Just boot into your BIOS and I dont recall what section it is under off hand: "sata configuration", "onboard devices", or something. Your choices will be IDE, RAID, or AHCI. choose AHCI.
AHCI. Make sure to run the Intel auto-update thingy from their site so that you have all the latest USB, southbridge, etc. drivers. From what I can remember, I also had to install Intel Rapid Storage for my C300 to function properly.
__________________ For my part I know nothing with any certainty, but the sight of the stars makes me dream.