Windows.old contains all your User information carried over from your previous Win 8 installation. It was put there in case the installation went wrong so you would not lose all your data. When things have settled down and I am comfortable with the new 8.1 it is my plan to uninstall it. In the meantime I consider it a useful extra backup for a week or so and I am planning to leave it for a while. You may also find this from Microsoft to be useful.
As far as I know there is no 8.1 download site so that you can save it and burn it to a disk, if you find one let us know.