Hi there
This can also occur if you attempt to activate something "automatically" after dong an install with a product downloaded from technet or elsewhere (legal folks please).
What you need to do is AFTER install go to CHANGE PRODUCT KEY and enter the KEY you got from technet (or your supplier).
Now re-try activation and it should work.
This might not be your problem but I have had this when trying to activate "automatically" after installing products I've got from technet.
Cheers
jimbo