I believe it is AHCI and not ACHI. RAID is only possible if you have more than one physical disk drive. Anyway, follow the instructions here to enable AHCI mode after OS setup.
Windows 7
http://www.windows7news.com/2010/05/25/how-to-enable-ahci-in-windows-7/
Windows XP
For Intel chipset – download and install Intel Matrix Storage Manager. For other chipsets, look for relevant download sites.
Guide:
http://expertester.wordpress.com/2008/07/27/how-to-enable-ahci-windows-xp/
