If you don't know emacs and you already have Word you could use the outline mode of it. I have used it in the past and it works pretty well. I haven't sat down and learned emacs so I have yet to try org-mode but I hear that works well too.

I have yet to find a truly great outlining program yet. (Note: I plan on learning org-mode emacs but I am a vim guy so the initial inertia of learning it is high.)