Originally Posted By: Cornet Nev
OK, lets just run you all through the Microsoft license business first.

Firstly, any ready made computer manufactured by any of the big names will usually have what is called an OEM license for the operating system on it. You could also as an individual buy an OEM license. However these are a little cheaper than a full license, they were created to be used by system builders, not really for the Joe Blogs such as us. Being cheaper an OEM license is not in any way transferable, so cannot be installed on another computer if the first one dies. The license dies with the computer.
If however you were to buy a full retail license, and then your computer died, you can then install that operating system on to a new computer, providing of course that the old copy is removed from the old computer, or is not operational in any way. It is still one license for one computer.
Up until now, that has always been the case right through from the very first Windows operating system. However, it would now seem that the OEM version will be no more with Windows 10. What exactly this will mean as far as transfer, dual booting, and other things is yet to be seen.
However. going back to the previous operating systems, providing you buy the license for each operating system you intend to run, there is nothing to stop you from dual booting say Windows XP and Windows 8.1 if you have bought a license for each, even if one or both of them happens to be an OEM version.
Considering these are still supported for at least critical updates, you could buy OEM licenses (Providing you can still find them on sale) For Windows Vista (SP2), Windows 7 (SP1) Windows 8.1 and run them as multiple booting on one computer. Obviously the same would apply if you bought full retail licenses. You just have to bear in mind that if the licenses are OEM versions, they live and die with the computer they are on and cannot be transferred.
The important thing is that the license conditions do not allow you to run the same copy on two different computers. There is a way, however you must then buy the multiple license, which of course costs even more, and is not normally available outside a business environment.

So, you now have for example a computer running Windows 7, or Windows 8.1, it does not matter whether it is an OEM license or a full retail license, and you then upgrade via the upgrade system to the new Windows 10. Microsoft state that the original license for the previous operating system is transferred to the new upgrade. If you kept a copy of the old Windows 7, or 8.1, it is no longer licensed, unless you go through the previously mentioned downgrade back to your old system, and therefore scrap the Windows 10 copy. You only have 30 days to do that as the old system will be automatically deleted after that time.
That also means that any clone, system image, or other copy of the previous system will also be out of license after that 30 days, regardless of how you remove Windows 10.
If however you do wish to dual boot or even multiple boot with Windows Vista, Windows 7, Windows 8.1, and also once released Windows 10, you must still buy the license for each, you cannot for example upgrade the Windows 8.1 to Windows 10 and still run the old Windows 8.1, at least not without buying a Windows 10 license and installing that without removing the original Windows 8.1

I know it all sounds very complex, however that is the way Microsoft works, they are only trying to keep their copyrights managed and prevent piracy.

Win 11 64, Asus Rog Strix z390 mobo, 64 gig RAM, 8700k