Moving my work laptop to Windows 8 goodness, and the key embedded with the install media wasn't working but I had been given a valid one from the IT Dept. - a quick surface Google/Bing kept suggesting to click on the "change product key" option, but the link doesn't appear for me in this instance. What to do?
I came across this post (for Vista, would you believe it), and the instructions work. Basically, open up a command prompt in administrator mode, and type:
Note that it's /ipk not -ipk in Windows 8.
Then, you should be able to refresh the System window, see that the key updated to your correct one, and hit activate and run away laughing!
Note that the error message I had when it was trying to validate with the old invalid key was "DNS not found" or something along those lines - which is a lie; don't be fooled!