20,000 words for a Word document is trivial (Word 2003 can handle 32Mb of text, plus graphics, etc.). Perhaps your Office installation has a fault, or your document has some kind of corruption.

Try repairing the Word installation (via Help|Detect & Repair).

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.

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Cheers,
Paul Edstein
[Fmr MS MVP - Word]