Windows Vista Product Key Already in Use

[Unregistered]

I have a system builder's copy of Vista Home Basic. I did build my own computer so that should not be an issue. I needed to upgrade my hard drive so I backed up all my info and reformated the old drive to use as an external. The installation on the new drive went well however Vista will not activate it tells me "The product key you typed is already in use". I attempted to contact microsoft by email. When I enter my product key it tells me it is not valid, I have verified it and it is correct! Any ideas on how to fix this? I can't call the 800 number since I am currently overseas.