I'm afraid you're totally wrong mpilchfamily. To use RAID/AHCI, drivers must be installed on installation, because the boot sectors are modified for this functionality. You can do it once you're in XP, but it's a bit risky because you could lose access to your boot drive if you screw things up. I have not done it so I can not provide a description but if you google around you may find it.
You have to use a floppy because that's the only thing XP accepts - you can't get them off the CD-ROM! There is usually a "makedisk" type program on the CD-ROM that will create such a floppy mode, and then you need to know the exact driver to install (ICH8R, ICH8, etc on Intel-based boards).
If you are installing Vista, you can though. As long as your media is formatted FAT32 you can get those drivers from hard drives, USB keys, etc.
You also have to remember to change your BIOS for AHCI/RAID. All BIOS default to IDE, so you need to have that set first before installing XP.
I highly recommend you read Intel's Matrix Storage Manager documentation on their website, even if you aren't using an Intel chipset, as it provides extensive information and explanations.
As has been said... you need to do it during setup. I had to download the driver for a Gigabyte S3 board. It's just a few files that go on a floppy. You'll probably be presented with a few options when you put the disk in... Make sure you choose the right one! The manufacturer's website usually has the information regarding which one to choose.