Improving Windows 7 performance by enabling AHCI mode

Introduction  I recently upgraded a user from XP to Windows 7. After the installation I realized that the machine was running terribly...

Introduction

 I recently upgraded a user from XP to Windows 7. After the installation I realized that the machine was running terribly slow. The start up would take 5-6 minutes (sometimes more).

I tried to run all sorts of diagnostics tests to pinpoint the bottleneck. I tried checking the Resource Monitor but that did not give much info. I even installed Microsoft Administrative tools and another tool called Latency Monitor to try and get the exact service which was causing the sluggishness but to no avail.

Finally when I was just about to give up and rebuild on a new HDD a senior suggested that I try and check the SATA mode in BIOS. When I did, I saw that AHCI mode was disabled and the machine was running on RAID. I read that Windows 7 performs much better with the AHCI mode enabled so I did. But that did not work as the AHCI drivers were not loaded at time of installation. The system would not recognize the HDD and I would get the Error: 'STOP 0x0000007B INACCESSABLE_BOOT_DEVICE'. The cause is Windows 7 will only install the AHCI drivers at installation if the mode is enabled at the time of installation. So I booted in RAID mode and manually enabled the AHCI driver in the Windows Registry. This process is as follows:

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 one of the following registry subkeys:

HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\Msahci

HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\IastorV

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.

The MIcrosoft KB link for this is here: http://support.microsoft.com/kb/922976

I restarted and went into BIOS. Changed the mode to AHCI and this time the boot was successful.

The machine performance improved drastically and we all all lived happily ever after! (Till the next breakdown that is!)