What all disks do you have, and what ports are the connected to (Please tell us color and #)
To change from IDE to AHCI or RAID after you've already installed windows you need to make the following changes
Enable switching between all IDE/AHCI/RAID modes by changing "Start" Values in these keys to 0 (Win 7 / Vista ONLY)
HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\Msahci\Start
HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\Pciide\Start
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\services\iaStorV\Start