You can also slipstream the AHCI drivers into the XP ISO and install the OS that way. It is tedious and took me 3 different burns to get it right, but it does work.

I believe there is a program that uses your XP install disk and your specific motherboard AHCI drivers and will create the ISO. You then burn the disc and install.

I wonder if a USB floppy would work?

Depends on your bios but it should. On my board it would, but I decided it wasn't worth the extra effort. The system is so lean I can just turn the thing off and unplug the drive.