As Charles indicates, your document may have acquired some form of corruption. If one or more of your tables is the source of that corruption, the 'repair' method he describes won't fix it. Unfortunately, too, Word has no tools to help with identifying which table(s) might be corrupt. Corrupt tables can often be 'repaired' by:
• converting the tables to text and back again;
• cutting & pasting them to another document that you save the document in RTF format, which you then close then re-open before copying them back to the source document; or
• saving the document in RTF format, closing the document then re-opening it and re-saving in the doc(x) format.
Do note that some forms of table corruption can only be repaired by converting the tables to text and back again.

Before you go down that path, though, I'd suggest repairing the Office installation (via Windows Control Panel > Programs > Programs & Features > Microsoft Office (version) > Change > Repair - or whatever the Control Panel path is on XP). Do also take heed of Charles' advice re the incompatibility of Windows XP and Office 2013.

__________________
Cheers,
Paul Edstein
[Fmr MS MVP - Word]