Even if you imaged the old HDD and transferred its contents to the new HDD, it is possible that your HDD replacement has indeed triggered Office's activation security. Installing Office afresh on the new HDD would necessarily require re-activation; otherwise its trial period would expire after a month.

If you purchased Office on-line, you'll have been emailed a Product Key, otherwise it will have been printed on the case the installation media came in; without it, you will be unable to re-activate Office.

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Cheers,
Paul Edstein
[Fmr MS MVP - Word]