Hello Dibbz, and welcome to Ten Forums.
If this is for the free Windows 10 upgrade offer, then it must be upgraded from your activated Windows 7/8.1 PC first to register the PC for the Windows 10 activation. Afterwards, you can do a clean install of Windows 10 on the PC if you like.
Can I reinstall Windows 10 on my computer after upgrading with free upgrade?
Once you’ve upgraded to Windows 10 using the
free upgrade offer or
Media Creation Tool on an activated Windows 7 or 8.1 PC, you will be able to reinstall, including a
clean install, on the same device. You won’t need a product key for re-activations on the same hardware. If you make a meaningful change to your hardware, you may need to contact customer support to help with activation. You’ll also be able to create your own installation media like a
USB drive or DVD, and use that to upgrade your device or reinstall after you’ve upgraded.
You do not get a product key with the free upgrade of Windows 10.
The activation for the free offer of Windows 10 is registered on the Microsoft activation servers, and is referenced by the hardware (motherboard/CPU/graphics card serial numbers) of the PC it was upgraded on.