MAJOR EDIT 2015-08-02 I am grateful for the folks that have posted here to correct my outdated views of how Win10 is activated and stored. My age is showing and what used to be the way I remembered how MS protected its OS from being misused and how it can remember that my computer has a valid license has changed a LOT since the days of Win95 and XP. I will highlight the changes but leave my mistakes for others to comment on.
The confusion that most people are having is what they are getting with the Win10 upgrade. Too many people are assuming that MS is giving away free copies of retail Windows 10. That is simply not the case. MS is letting Win8.1 users patch their Win8.1 computer to make it run a a copy of Win10 with very limited rights. If you want all the rights that a retail version has, you need to BUY a copy of the full retail Win10 Pro product. Don't confuse the two, they are different in what you can do with them. EDIT This statement is simply wrong. If you start with retail, you end up with Retail Win10. If you start with OEM, you end up with OEM Win10. End EDIT From another of my posts:
Think of the Win10 upgrade as simply an update patch on top of Windows 8.1 to make it look like Win10. Lets call it Win8.1++. Don't think of it as a completely new install of Win10 because it isn't (EDIT Ay-yi-yi. I really blew this part. In fact it is the same as the retail version of Win10 if you started with the retail version of your old OS, OEM has the same restrictions End EDIT). It only has a generic product key and it does not have a way of doing a clean install by just using a Win7/Win 8.1 product key on a new computer (EDIT that is still true as far as I know. Some people have said that they have successfully activated a clean install on top of an OEM version of Win7 without first doing the upgrade to Win10- I simply cannot verify this End EDIT). You do have the ability to do a clean install of Win10 on the upgraded computer, but only on that upgraded computer. Like any other retail copy of Win8.1 that you want to move to a new computer, you first have to do a clean install of Win 8.1 then run updates on it including the one to make it LOOK LIKE Win10 (Win8.1++). Of course, you need to wipe the old copy of Win8.1++ off of the original machine as well. Simple enough. Oh, and you only have a year to play with it before the win10 update patch for Win8.1 that turns it into Win8.1++ goes away so no more transfers to a new computer after that (unless MS once again changes the rules or make exceptions).
If you want a real, complete retail version of Windows 10 with all its rights and privileges including the ability for a clean install on a new computer, then you have to buy Windows 10 Pro for $199 which will give you a unique, valid Windows 10 product key. EDIT True Enough. Of course a retail version of Win8.1 can also be moved to a new computer but what happens after the year's free upgrade, I donno. End EDIT
EDIT here is a major change from why I thought I knew. After a Win8.1 to Win10 upgrade has been done, MS creates a hash code for your computer and stores it on their servers. You will never have to enter a product key again for either Win8.1 or Win10 on that computer for a clean install. End EDIT