how to identify network activity?

Hi there,
I have a computer that is not a server. I mean it should have no network activity as it is meant to display videos and not to provide any ftp, web or whatever service.
The probleme is that the computer actualy reports a constant network activity.

How can I identify the processes that are using the network?

Thanks for your help
Santiago

As root on Linux:

# netstat -anp

In the next-to-last column look for ESTABLISHED (won't appear for UDP). The last column gives you the PID and process name. If it's empty/'-' even as root, it's probably a mount.