If u r not concerned with bandwidth., you may install dropbox in windows with its own default dropbox folder location. Files saved on mac dropbox will be synced in windows dropbox folder and same is true vice versa. in such a setup Dropbox will consider vm as separate system and sync data.
I personally prefer making use of shared dropbox folder as suggested in earlier post. It not only serves the pupose of backing up data on mac but also syncs up data across all of my other systems.