That depends if the key was bought for ten dollars (they are no longer sold) or if it was obtained for free from MS during the brief period after Windows 8.0 was released and people could sign up with Microsoft to get the Media Center Key for free.
If it was bought for $10, then you might be still able to use it to activate the Media Center.
If it is one of the free keys that was given out, then you cannot use it now for the first time because they had a deadline by which they were supposed to be initially activated.
I found this out the hard way because I used my emails to sign up and get those free keys but they only activated during the promotional period, and could not (initially) activate after that.
Microsoft does *not* want people using Windows Media Center any more.
It's a matter of speculation on why, but they do not want you _easily_ recording TV - that's something that can _easily_ be done only with Windows Media Center, incredibly, and no other software... other software as you know requires jumping through multiple hoops.