Let's not open another discussion if that's right or wrong but if you don't register there's usually bad news as you ask MSFT to correct the key issue.
No one can tell you if you were hacked or other. I'd register ASAP.
I've had my desktop PC for about 7 months with Windows 8.1 installed and use it pretty much daily. Suddenly, it just gave me a message informing me that my product key is being used by another PC. Recently I've been worried about the lifespan of this hard drive, so I downloaded Belarc Advisor, got my Windows product key and copied it onto a passworded file on an external USB drive which I keep unplugged when not in use. Is it possible that I got hacked and someone stole my key, or is this a common error? I downloaded the advisor and made the transfer for safekeeping about a week before Windows gave me the message. I'm very careful about Internet safety and use Bitdefender. If it's not a virus/hack, what can I try to fix this? It doesn't seem to be that urgent of an issue as Windows still works just fine, but will it stop working at some point if I don't register it? Any help would be much appreciated.