If you have installed Windows while your motherboard SATA controller was set to IDE mode, you can not switch back to AHCI/RAID mode, easily. If you do so, you can not boot in to windows and you will get BSOD for sure. The typical solution to this, Is to prepare a SATA AHCI/RAID driver diskette and do a reinstall of windows with F6 key procedure.
Ok, now you can enable AHCI/RAID mode (ICH5...ICH10) and avoid reinstalling of windows. Here it is:
2.Be sure that the Bios settings for SATA ports is set to IDE mode.
3.Boot in to windows.
4.For AHCI mode, import "Intel_AHCI_Merge.reg" And for RAID mode, import "Intel_RAID_Merge.reg" in to the registry. Just import the one that is suitable for you.
5.Copy the Included "iaStor.sys" to "system32\drivers" directory. As of writing this, This is the latest(v126.96.36.1993) AHCI/RAID driver from Intel.
6.Reboot your system, go to Bios setup and enable AHCI or RAID mode for SATA Ports.
7.Boot to your windows and you are done.
Now Optionaly, You can download the latest Intel Matrix Storage Drivers form intel site and install/update the AHCI/RAID drivers.
Note that, Intel does not support pure ICH8/ICH9 chipsets that are set to AHCI mode, and does not provide AHCI drivers for those chipsets. Asus P5B Motherboards are examples that are affected by this situation. But with this package you can enable and use AHCI mode with pure ICH8/ICH9 chipsets that have AHCI support enabled through Motherboard Bios.
Update (April 13 2010):
-Updated the driver to v188.8.131.524
-Fixed an error in the reg files that could prevent the driver to operate on some systems.
For Vista and newer MS OS