The Advanced Host Controller Interface (AHCI) needs to be enabled in the computer’s bios. Windows 7 will not load if this is done without ensuring that the AHCI driver is loaded in the operating system (which it is not by default). The benefit of enabling AHCI is that it will speed up read and write operations of the hard drive. (This will occur only if the hard drive supports AHCI.) It is therefore recommended to check the hard drive first to ensure that it is supporting AHCI.
Here is a step by step guide to enabling AHCI in the Windows 7 Registry.



* Press (Windows R), enter (regedit) and hit (enter).

* This opens the Windows Registry editor.

* Move to the path HKEY_LOCAL_MACHINESystemCurrentControlSetServicesM sahci

* Locate the parameter (Start) on the right side and double-click it.

* Change its value from 3 to 0

* Restart the computer and enable AHCI in the bios

Windows will install the AHCI drivers on the next start and prompt for another reboot afterwards.
Remember be always careful while changing settings inside the registry and the bios. Otherwise, you computer can even won’t work again. Just be careful.