In case you hadn't already heard Windows 10 (before you ask, there is no Windows 9) will be a FREE upgrade to almost everyone. Those of you with Windows 7 or Windows 8.1 will be able to upgrade your computer, for free, to Windows 10 when it arrives this coming fall. Vista users are out of luck and XP users .... what are you still doing using XP!!!! It's not secure people! But I've mentioned that in a previous post.
The only caveat is that your existing computer must be able to run Windows 10 (and if you are already running Windows 7 or 8.1 then it will run Windows 10) and that you have one year from date of launch to do the free upgrade.