How to enable AHCI without reformating.

Most current notebook nowadays allow user to set hard disk interface either AHCI (advance) or IDE (legacy). There are several reason why we should use AHCI instead of IDE. One of AHCI advantage is NCQ (Hot swappable seem not so relevan to notebook aight).
The Advanced Host Controller Interface (AHCI) is a hardware mechanism that allows software to communicate with Serial ATA (SATA) devices such as host bus adapters which are designed to offer features not offered by Parallel ATA (PATA) controllers besides higher speeds, such as hot-plugging and native command queuing. en.wikipedia.org/wiki/AHCI

AHCI stands for Advanced Host Controller Interface. One of the main reasons for switching to AHCI-mode is to take advantage of the NCQ-Feature of your SATA harddrive. NCQ (Native Command Queuing) allows ATA drives to accept more than one command at a time and dynamically reorder the commands for maximum efficiency. NCQ, when used in conjunction with a hard drive that supports NCQ, can increase storage performance on random workloads.
forum.msi.com.tw

Native Command Queuing

Native Command Queuing (NCQ) is a technology designed to increase performance of SATA hard disks under certain situations by allowing the individual hard disk to internally optimize the order in which received read and write commands are executed. This can reduce the amount of unnecessary going back-and-forth on the drive’s heads, resulting in increased performance (and slightly decreased wear of the drive) for workloads where multiple simultaneous read/write requests are outstanding, most often occurring in server-type applications. However, the current technology actually slows down HD access in certain applications, like games and sequential reads & writes, because of the added latency induced by NCQ logic. en.wikipedia.org/wiki/Native_Command_Queuing

AHCI is fully supported in Microsoft Windows Vista and the Linux operating system from kernel 2.6.19. Older operating systems like Windows XP require drivers written by the host bus adapter vendor in order to support AHCI. Windows XP requires the installation of a vendor-specific driver even if AHCI is present on the host bus adapter because Windows XP was released before Serial ATA was invented.

So, how to enable AHCI if you already install Windows Vista using IDE interface.

Solution…

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_MACHINE\System\CurrentControlSet\Services\Msahci
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

Reboot your notebook, enter bios (F2, del etc), change your hard disk interface to AHCI.

Guide on how to change AHCI setting thru bios : http://komku.blogspot.com/2007/08/windows-xp-and-acer-aspire-4520.html

Credit : Komku blog + Microsoft Support Page