I have a genuine Windows 7 machine. Can I buy the £24.99 Windows 8 upgrade and install it on a new desktop I bought with no OS?
Will it work on my Windows 7 machine, even if it’s not genuine or is this just not possible?
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Owning windows 7 gives you the right to purchase the cheaper upgrade, but does not necessarily require that you upgrade the Windows 7 machine that gave you the right to make the purchase. You can apply the upgrade to any machine.
However, it is still just an upgrade. You still need an existing valid Windows operating system of some kind on the target machine to "anchor" the new Windows 8 installation on the PC where you will use the upgrade.
The Windows 8 upgrade license is not valid by itself. It must have another license to "merge" with on a single machine in order to be valid.
Assuming that you have a non OEM version of Windows 7 then yes you can uninstall Windows from the old machine and then install Windows 7 on a new machine and then install the Windows 8 upgrade. You may have to contact Microsoft to get the Windows 7 activated properly if it decides the new machine is too different from the old one.
You will more than likely encounter problems if you try to use the old machine with Windows 7 still installed as the Microsoft servers will assume you have pirated it as it will be "active" on more than one machine.