You didn't mention your OS. If you have XP, the only way to switch from IDE mode to AHCI is to reinstall XP. You will need to use the F6 option during installation to install the AHCI drviers.
Win 7 has native AHCI drivers, but if you installed it when the bios was in IDE mode, switching to AHCI mode won't work. There is a registry hack to fix this with no need to reinstall the OS.http://support.microsoft.com/kb/922976