Thanks for the response. This machine only has a single hard disk and single DVD drive. Any other ideas? I've tried two different hard disks. What I find odd is that an unmodified Windows XP 64-bit install will still see the drive with the controller in AHCI mode. Maybe I need to try it again with the Intel driver, but re-do the partition table on the disk.
Has anyone successfully installed Windows XP Pro 64-bit on a P55 based motherboard? (Intel DP55WB) I get through the text mode install (with or without adding the Intel SATA drivers to the disk), but after rebooting for the graphical install, the machine is unable to find a boot device. Setting the BIOS to AHCI or IDE mode does not seem to make a difference.