Does your router have a statistics page? If it does, you can check to see which machines are out of line, bandwidth wise. A simple method is to just periodically refresh the page to see the updates or you can log all the information off to another machine, if your router supports this. You can also use a network monitoring tool, such as the one from Microsoft (
http://www.microsoft.com/download/en...ang=en&id=4865) or others mentioned in this thread.
Note that you should also check your firewall logs to see if part of your bandwidth isn't going to hackers trying to break in to your system. If this is the case, your provider should forgive that bandwidth because their upstream protection isn't working properly.
Finally, a real simple check is to shut down machines when not in use. It's a very gross technique, but can give you a good feel for what is happening. You can use this technique as a process of elimination as well.