Is the 7.2 installation running? If so, I'd think the best, safest, and supported route would be to upgrade it to 8.1 in place, and back that up using the backup option from the console to an attached USB drive. Then install 9.0--if you really want to be on the safe side, install it to a new drive, so if something goes wrong, you can easily revert to your old installation. When you boot 9.0, one of the first things it will ask is whether you want to restore from a backup. Answer yes. This should do it, though I have no idea what you mean with the reference to a "Windows machine" in your post.
This will restore all your system configuration settings, your users, their data, your ibays, etc. It will not restore any contribs or non-standard software you may have had installed.