Have you tried repairing the Office installation (via Start > Windows Control Panel > Programs > Programs & Features > Microsoft Office (version) > Change)?

It's also possible the document you're working with has acquired some form of corruption. Corrupt documents can often be 'repaired' by inserting a new, empty, paragraph at the very end, copying everything except that new paragraph to a new document based on the same template (headers & footers may need to be copied separately), closing the old document and saving the new one over it. Similarly, corrupt tables can often be 'repaired' by converting the tables to text and back again or by saving the document in RTF format, closing the document then re-opening it and re-saving in the doc(x) format.

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Cheers,
Paul Edstein
[Fmr MS MVP - Word]