Create a new area that uses the Sites Directory template and call it SiteDirectory. You can then go into the settings for the area & rename it to Sites.
Be warned, though, that SharePoint may create the new area under a "C" folder, e.g. C1/SiteDirectory. I don't know if it is possible to move an area from that path up to the top level, which is where it needs to be in order for certain bits of SharePoint to work. There is a KB article that talks about how some parts are hardwired to the specific URL.