What is AHCI and how does it differ from SATA IDE ...

AHCI stands for Advance Host Controller Interface. It is a newer technology to provide advanced features to the Serial ATA standard. It allows for access of the native functions of the SATA interface such as Native Command Queuing (which allows greater performance by optimizing how controller requests data from the hard drive) and hot swappable of SATA devices so that you may remove SATA drives from your PC without first having to shut down your computer.

If you are installing Microsoft's Windows Vista or higher, AHCI is directly supported natively on the operating system installation DVD so you do not have to load AHCI drivers during the installation of the operating system.  However, Microsoft's Windows XP does not include native support and therefore you will need to manually install the AHCI drivers during the installation of Windows XP.  If you do not feel comfortable installing AHCI drivers, you may use what is referring to as SATA IDE Compatibility Mode which can be triggered in the motherboard BIOS.  SATA IDE Compatibility Mode disables AHCI however it will allow you to install older operating systems such as Microsoft's Windows XP without the need to install AHCI controller drivers.  Unfortunately once you have installed the operating system, you may not switch modes from SATA IDE Compatibility Mode to AHCI or vice versa without reinstalling the operating system.