In Word2007, all you have to do is be careful to start the sub-chapters with a different heading style. Then use the chapter heading style for numbering your figures. Word then ignores your sub-chapters when it numbers your figures, tables, and equations. Oh, and you have to have the entire document set up as a multi-level list, but it sounds like you've already done that.
p.s. Although this is a great question, and I learned something answering it, it doesn't really belong on StackOverflow since it isn't a programming question. Someone will suggest a better StackExchange site for it. (Super User?) Don't get mad! It is very easy to open accounts on other StackExchange sites. Your logon credentials are the same (unless you make them different). Your reputation doesn't carry over to different sites, but your rep here is only 23 at the moment, so no big deal there. ;-) If you move it to Super User, leave a comment here to that effect, and I'll come answer it over there. Then you can select my answer, and I won't be a 6 there anymore. LOL! And maybe you can answer my one question over there.