Reactivating Office on the same PC doesn't use up a licence unless there is a significant hardware change. If the new HDD is the same make & model as the old one, it shouldn't count as such a change. Even a different HDD make/model might not count. Regardless, the worst this will ever mean is that you might use up all your automatic activations, not any installation licences. If that happens, you'll need to use telephone activation, that's all.

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Cheers,
Paul Edstein
[Fmr MS MVP - Word]