At the moment if the BIOS isn’t ACPI compatible then XP Pro 64-Bit will not install. You will have a better chance getting Windows 2003 Server 32-Bit running on it if you wish to use more than 4GB of RAM installed. Otherwise stick to Windows XP Pro 32-Bit if you only plan on running regular programs and not memory intensive.
Now you mention your X99 motherboard. Does your XP Pro 64-Bit install on that without any issues?