I reckon you've got a SATA hard drive. You need to obtain the SATA driver for your motherboard, and install it DURING the XP setup:
Copy driver to floppy disk
Reboot the PC with the XP disk inserted.
XP setup will begin
When the blue screen appears press F6 to install a 3rd party driver (this happens very early in the process and you have little time to action it, so watch closely)
XP will search the floppy for the driver, and hopefully load it.
The disk should then be available.
Colonel Blinky, April 2005