Final Fantasy XIV requires 12GB of free space in the drive where “My documents” is. If you are like me, and don’t have that space available you can do the following to change the default location of your my documents folder available
1.Open my computer and browse to c:\users\yourname\my documents
2. right click the my documents folder, select properties, and then select the tab location. Choose a new location with plenty of space, then hit OK.
That’s it, your document will move and you will be able to install the game.
Actually this is kinda stupid, because instead of writing the error message, they could have added a couple of code lines to allow manual selection of the temp folder for the downloader.