Monitor traffic and internet user in local lan

I’m looking for a windows software solution to monitor internet traffic on a
lan. There are several pc’s connected to a ADSL Broadband Router gateway. I already tried
dumeter, analogx and bommalarm but they only monitor the total traffic on
the gateway. I need to know the individual traffic for each pc so i can
charge the user behind his pc for the down/uploaded bytes.

Any suggestions are appreciated!