I am having the same issue with XP and Office Word 2007
When this happens, Save As... with the same name in a different directory - I usually just go one level up - then exit Word and move the file back into the original directory, overwriting the original file.
This is also a good technique to use on Word or PowerPoint files containing pictures, as it often reduces the file size considerably.
I am trying opening the file from within Word rather than from Windows Explorer. So far, so good.