Clean install of OS(operating system)

[Raist]

Just the main client folder where you install the game is pretty much enough. I even copied it to my external HDD so I can play FFXIV at any place where there's a PC (usually internet cafes).

Yeah.. I just wanted to demonstrate how it works when you have the previous patched files already on your drive. This way you can avoid having to download all that crap again after you install. Some people are having issues staying connected through the long download process. With a little advanced preparation, you can bypass most of that downloading--but it will require a lot of space to backup all the data.

[combatFlexo]

Successfully upgraded to Windows 8.1 and have FFXIV installed after backing up the game files.

I backed up the following folders:
C:\Program Files (x86)\SquareEnix\FINAL FANTASY XIV - A Realm Reborn
C:\Users\USERNAME\Documents\my games\FINAL FANTASY XIV - A Realm Reborn
and FFXIVSETUP.exe

Once I had Windows 8.1 installed I ran the FFXIVSETUP.exe it recreated the C:\Program Files (x86)\SquareEnix\FINAL FANTASY XIV - A Realm Reborn folder and a shortcut on my desktop.
I copied over my backup folders to the original location and launched the game. I was able to play without any updates and my user settings were the same(HUD, keybinding, etc...).