How can I find what's using the internet?

I have DSL through AT&T. I have the modem wired into a wireless router, and the router is encrypted so nobody is using it. The one thing that constantly bothers me is that my computer shows that something is using bandwidth. This happens when I'm not in any program, and is starting to worry me. Is there a good program that can show me every process that's using bandwidth and close the ports they listen through if necessary?

 

Win all includes Vista

http://majorgeeks.com/TCPView_d599.html

I use this on on XP pc

http://majorgeeks.com/Active_Ports_d682.html

 

Your firewall's active connections list should show this info and allow you to terminate or block any unwanted activities.

If your system is clean, it is more than likely one of your programs checking for updates.

 

If you have Vista, go into the Task Manager, and under the performance tab should be Resource Monitor. Click that, and there should be an option to expand the Network bar which tells you exactly whats accessing.

http://img12.imageshack.us/img12/5264/resources.jpg