Because a product key or a Certificate of Authenticity was more common with old operating systems or software, the tag product-key would be a good key to add. The excerpt it could have:
A product key helps prevent piracy of software. Use if you have a question about product keys and old OS's or software.
or
For questions asking about product keys and old operating systems or software.
Picking up Stephen Kitts comment, I think a tag that covers copy protection schemes/keys is more apropriate, as it may be useful not only for MS-stuff, but anything from installation keys to page numbers on manuals asked for in a game.
This seems like a useful tag to have. However, I'd be tempted to call the tag license-key instead... though I'm not sure why. I have a gut feeling that it'll be applicable to more questions, but I can't think of any examples.
I am not so sure about that - MS introduced product keys with Windows 95, started to work with them in earnest with Windows 98 by ~2000. This is somehow on the borderline of what I would consider "retro".
A product key program that was really working required an internet connection and online activation. Only after that was feasible, product keys started to make some sense. And the Internet as a widely available commodity is definitely non-retro, IMHO.
