Hi Lwaxana,
Lots of good ideas and options here, but I think Kat had the best option in the simplest form. A firewall such as Kat suggested would pop up a flag (little window in the corner of your monitor) each time something tried to access the Internet. It allows you to decide weather to allow or deny the action. You'll see a reference on that pop up window as to what file is trying to access the Internet. From that you can research and reference that file (back track) to see what application it's related to and either remove the offending application or change settings to stop the automation in most cases. Not fool proof, but should tell you what's going on for the most part with your system.
Another way, although time consuming, would be to open the 'Run Box' from your start menu and type "msconfig" in the box then hit OK. This will open the configuration dialog for your system (be careful what you mess with). Click on the startup tab and you will see all the things that start/auto-load when you boot your system. There are check boxes next to each event. You can clear these check boxes to stop that particular file/app from starting. Do this one at a time in an effort to track down which one may be the culprit. Then close the box and restart your PC. Watch things and if your still having trouble, then reopen the dialog as above, check the box you cleared, and chose another to clear and try. It's time consuming and not a 100% guaranty that you'll find what your looking for because not all background services that can be accessing the Internet will be listed or show up there.
Hope that gives you some direction towards your need to find what's taking up your bandwidth. The other options/software referred to here may be beneficial as well. I haven't tried them so can not comment. I do suggest go with the firewall first and go from there. Good firewalls of this nature can log all the Internet activity/access too so you should be able to review it and figure out what's accessing when. Good luck!
Regards...
Edit:
Here's a free application that will list the processes active in real time called > Process Explorer.
MS help page to understand system internal processes.
A list of application utilities to help track things down can be found by clicking > here.