So I downloaded and installed the Game Client fresh onto my main W7 Pro 64 bit computer and then transferred the game files over to my secondary computer running the W8.1 64 Bit RTM trial on a USB thumb drive and everything seemed fined.

After clicking PLAY it reported there was a missing DirectX9x.dll so I downloaded the DirectX end-user DirectX runtime and after that it seems fine. There was no repair option available in the launcher but I was able to get through to the log in for the game client.

It's possible I may need to run the Repair option when/if it becomes available in the launcher.