Hi Paul

The style you're using (probably Normal) has probably been set to update
automatically.

In versions of Word before 2002, Format > Styles > Modify. Un-tick
Automatically Update. Tick Add to Template.

In Word 2002, Format > Styles and Formatting. In the Styles and
Formatting task pane, a style will be highlighted. Right-click on that
and choose Modify. Un-tick Automatically Updated. Tick Add to Template.

If, when quitting Word, you're asked whether you want to save
normal.dot, say yes.

For an explanation of what's gone wrong and why this fix works, see
http://www.mvps.org/word/FAQs/Formatting/WholeDocumentReformatted.htm

Hope this helps.

Shauna Kelly.  Microsoft MVP.
http://www.shaunakelly.com/word
Melbourne, Australia