Bests & Worsts Reviews from Amazon

This is not a review of the Windows 10 OS, but of this specific product key and disc: I ordered this item to use the product key on my custom built PC. When I entered the key to activate Windows, I got an error stating "product key already entered," and would not allow me to activate Windows 10. This strongly suggests this is not a genuine copy of Windows and is therefore STOLEN or SOLD TO MULTIPLE PARTIES. I understand this may seem hyperbolic, but I just want to warn other people out there building your own PC--this will not activate Windows on your newly built PC booted from a USB, and this product is essentially defective. Very disappointing.