This is often the result of a font id conflict. All fonts on the Macintosh are assigned a font id, an integer value. When two fonts have the same id, some programs can become confused about the appropriate font to use. Microsoft word 4.0 used font id's to assign fonts, not their names. Since id's can be different on different computers, a word document's font could change when it was moved from one computer to another. Other signs of font id problems are inappropriate kerning or leading (the space between lines of text). Some font ID problems can be resolved by using Suitcase, which will reassign font ID's for you, as well as saving a font ID file that can be moved from computer to computer to keep the id's consistent. Font ID problems can also be solved with several type utilities, which will allow you to reassign font id's. Most newer programs refer to fonts correctly by name instead of id number, which should reduce the frequency of this problem.
This is the result of a bug in MSWord 5.0. The MSWord 5.0 updater, which can be found at the info-mac archives at sumex (in the demo folder), will fix this bug.