Microsoft Word 08 Formatting Problem

I've been having a problem with word 08 where every time I try to format a word as bold, the entire document is formatted as bold. When I undo, the document goes back to what it originally was, but the desired text is bold. It seems almost like word formats the desired selection and then does some sort of autoformatting that takes over the whole document, which is why I am able to undo it. Normally it's only a slight inconvenience, but now that I am writing my thesis, formatting is key, and I can't afford to have random formatting changes. Any ideas?