This may sound stupid, but have you made sure all the characters entered are correct? What edition of Windows 8 do you have, and what is the product key for?
Nope, I pasted the key from the website, so AFAIK it is correct.
OK, it's managed to activate itself..... I just rebooted the VM and it's now showing it as activated and as Windows 8 Pro.... :huh: Hey ho, ain't complaining!