Another possible option is to get a copy of the same version of your operating system and install that but use YOUR key
Well, good luck with that one.
While battling with Asus, I heard a rumor. Dell allow its customers to obtain that key. Don't know if it's true.
It's OUR BLOO&DY KEY! We paid for it, and MS should not have to subsidize* the repair of our machines because we have to do a clean instal.
*MS in Canada sent me a full 8.1 clean instal disc for about half the price of the cheapest net download. It means the computer has the key for the rest of its life . . . or, I can keep the software and still hack at Asus to do the decent thing.