How can I modify the spaces navigation?

I've been using xwiki for a week or so, and now I want to change the functionality a bit. After importing the recommended xar file, I see the spaces navigation on the right under the search field. Right now it shows all spaces (even those the user doesn't have permission to see) and all pages within that space. I'd like to change the navigation so that only viewable spaces and top level pages (those without a parent) are shown. However, I can't figure out what file or files I need to modify. I'm assuming that it's something in the /skins/xwiki10b1 folder. Anyone have any suggestions for me? Thanks, Eric