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Unfortunately not all (Microsoft) keys can be retrieved. Microsoft chenaged changed the way it "stores" keys for Windows and Office and not all versions store their key on the hard drive. Some versions of Windows 7 (Enterprise and Professional being two examples) do not record the key at all on the hard drive. They may use a service called MAK (Multiple Activation Keys) and/or KMS (Key Management Service) and/or VLK (Volume License Keys) These keys are non-existant and an attempt to retrieve them results in a string like "BBBB-BBBB-BBBB-BBBB-BBBB".
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