I have a 3,5 years old PC (first generation i5 2,66 GHz CPU, 4 gb RAM, Radeon 5850) and a year old laptop (2nd generation i7 2 GHz CPU, 8 gb RAM, Radeon 6650M). Both are running on Windows 8.1 Pro without problems. FF XIV works as it should, without strange crashes.

When 8.1 came out I upgraded from 2 fully updated Windows 8.0 installations. Since then only been updating Windows itself.

Only issues I've had with FF XIV was back at release with truckloads of crashes to the desktop. Especially when teleporting in the game. Issue was solved by updating the Catalyst drivers to the latest beta. Since then no more crashes