[AccessD] Monitoring net traffic to the internet

Can anyone point me to software that allows monitoring traffic to the
internet and tracking it back to the workstation?
 
My client has a very limited bandwidth to the internet and it is being used
in an unexplained manner, i.e. the bandwidth is not available to me (for
example) if I do an upload or download to FTP, response time is slow for
remote desktop etc.  We have seen this occur when one of the technical types
is downloading large service packs etc which we have arranged to have done
"off hours", but there are still times when the internet bandwidth is
abnormally low.  Thus we want to discover if someone is playing FM radio,
watching a video etc. just so we can go rap on their door and ask them to
stop.
 
I haven't a clue how to discover what workstation is using bandwidth like
this.
 
John W. Colby
Colby Consulting
www.ColbyConsulting.com