I discovered you can reuse a key off an old dead PC on a new one also (not that this is an option for the OP).
A lot of the pre-built's come with an OEM version that ties to the motherboard/parts when you first validate, so when it comes to validate the key on a new machine it won't go through.
But you can call MS and explain you're repairing/upgrading the original PC when you're attempting to do this, as long as the key is only going to be used on 1 PC they let you do it. Apparently half the time you just get an automated service when you call, and as long as you answer a certain question by saying "one" (PC) it will spit out a new key for you.
This was last posted about in 2013 when I was checking into it earlier.
Good news for me though, I'm in that situation now if my new PC order ships, going to need that OS a d already spent the budget for it.
FYI full retail discs of windows can be downloaded legally (search digital river) but you still need to get a legit key. I have it loaded up on a USB boot drive right now with the old PC key and waiting impatiently lol.