What typically happens is that you purchase the product key, then download Office from Microsoft. Beware, though, there are 'retailers' out there who are illegitimately 'selling' keys to corporate licences for Office. This allows the same key to be used for multiple installations. Be especially wary if the asking price seems way lower than retail. When MS finds out what's going on, such keys are liable to be invalidated.

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Cheers,
Paul Edstein
[Fmr MS MVP - Word]