AHCI  (Advanced Host Controller Interface) enables native command queuing and hot-plugging through SATA host controllers (Serial-ATA) for your hard drives. In many scenarios it enables more efficient multi-tasking. Vista was the first Windows OS to support AHCI out of the box, where as Windows 7 does the same. But an issue with AHCI is that if you install the OS without enabling AHCI in the BIOS, enabling it after installation will render your OS unusable. This is because Windows disable the AHCI driver since it is not needed during the installation.

There is one way to fix this, although you need to have knowledge of registry editing. The detailed steps from Microsoft are as follows:

To resolve this issue, enable the AHCI driver in the registry before you change the SATA mode of the boot drive. To do this, follow these steps:

1. Exit all Windows-based programs.
2. Click Start, type regedit in the Start Search box, and then press ENTER.
3. If you receive the User Account Control dialog box, click Continue.
4. Locate and then click the following registry subkey:

   HKEY_LOCAL_MACHINESystemCurrentControlSetServicesMsahci

5. In the right pane, right-click Start in the Name column, and then click Modify.
6. In the Value data box, type 0, and then click OK.
7. On the File menu, click Exit to close Registry Editor.

After this you’ll have to restart your computer, go to BIOS and enable AHCI. When you log in to Windows again, you’ll notice the installation of drivers for AHCI. Another restart will be required to finish the driver installation.

I’ve simplified the steps by Microsoft by exporting the registry key. Just download the following file and run it. You’ll still need to go through the steps of restarting your computer and enabling AHCI in the BIOS.

Hopefully, you’ll find a somewhat noticeable improvement in multitasking, specially if you’re a notebook user. Of course with the variety of PC hardware, experience many vary.

Disclaimer:

Beware though, you’re responsible on your own for messing around with the registry. This guide comes with no warranties if your computer blows up, gets angry at you, stops working or just shows a Blue Screen of Death.