My boyfriends laptop hdd just went kaput. Installed on it was Windows 10 (originally windows 8 but he did the free upgrade). Luckily I had a working hdd lying around from an old HP Pavilion dv7 that overheated on me a few years back. I put it in, it worked fine. I did a factory reset to clear up all the junk that was installed on it and now it's telling me "This copy of Windows is not genuine, Build 7600". Maybe it's because it was supposed to be for a HP laptop and this is a Toshiba, but that has nothing to do with the authenticity of the OS. I was thinking of just downloading Windows 8 and using the product key that came with the laptop, but of course all the writing on the sticker on the bottom has rubbed off (Why do they put the key there?!!!). Is there anything I can do to sort this problem out? I'm not buying anything as I shouldn't have to, I have two copies of genuine newly bought Windows sat on the HDDs. It's just that one HDD is broken and one is for a different laptop, that shouldn't matter, right?