Word 2000 Formatting

I’ve seen a problem, both at home and at work where formatted Word 2000 documents, when re-opened, no longer are formatted and, in fact, all of the “behind the scenes” formatting now shows up within the document text.
At home, I had to re-load Word2000 in order to keep this from happening. In the office, it mysteriously returned to normal.
Any suggestions or ideas as to what would cause this? I have fairly current virus software on both systems, so I don’t think a virus is involved.