It is also possible to use telephone activation, then explain the situation to an operator, and request an over-ride code. The response from MS does seem to depend on which side of bed the oprator got out of that morning, so be as nice as you can!
telephone activation (operator calls)
Click on the Start button
in the Search box, type
and hit the Enter key
follow the instructions - but when asked which service you require by the telephone ansafone, do NOT reply - this should force an operator to respond, who can deal with you.
Once activation is complete, and before you let the operator off the hook, go to the validation website at www.microsoft.com/genuine/validate
and check that everything is OK.