What Hawkeye22 and ubercake have been discussing makes a lot of sense, especially the fact the RAID systems are different from one maker to another, and hence require different things. I have systems with nVidia, ULI and Silicon Image controller chips. When installing Win XP that has no drivers for SATA, AHCI or RAID, one must either install one of those drivers with the initial OS installation, OR resort to setting the SATA Port Mode in BIOS to IDE Emulation to fool Win XP into thinking the SATA units are IDE. But here's the interesting wrinkle. On some systems the mobo support CD has separate drivers available for AHCI devices and RAID devices, so you do not install a RAID driver unless you actually are using a RAID array. Others seem to put the AHCI (or SATA) driver AND the RAID driver in one driver package, and call it just a RAID driver. SO that's what you must load, even if you are NOT trying to use RAID.
Now, of course, beginning with Win 7, the OS itself does have AHCI drivers "built in", so using SATA devices, even for boot purposes, does NOT require installing any extra driver with the initial OS install. And on those systems, if you later add a RAID array for data only, you may be able simply to install the RAID driver into an already-running Windows OS. That OS can load itself from a non-RAID boot HDD and then load from that same resource the RAID driver that allows it to use the RAID array. I can understand, however, that if the mobo driver package has both AHCI and RAID drivers together, installing that as part of the original OS Install will cover all eventualities for the future, even if you're not using a RAID array to boot from.