Okay, I've called Square about this and I've tried everything I can think of.
But..FFXI will not install on this Windows 10 system.
orinthia teamviewered in and couldn't get it to install either. Is there a way to manually install the thing? Grrr...
Oh and if anyone else wants to try to help, let me know and I'll load up teamviewer.
[Edit] The errors it's throwing are:
Internal Error 2711.EX567_ALL
Feature transfer error: Error -1603 Fatal error during installation. Consult Windows Installer Help (msi.chm) or MSDN for more information.