had same problem on some templates at work and it was linked to style. Can't remember which style was causing the problem, but if you change whoe document to Normal you shoudl find this doesn't happen.
This used to crop up frequently in Word97 and was all to do with styles and templates. In Word2002 (I don't have 2003), I see that it is in the Modify Style panel. There is a checkbox at the bottom right which says "Automatically Update". I am pretty sure that if this is ticked then any time you alter the formatting of existing text, the whole shebang changes as well.