How Can I monitor internet usage within my lan

I'm the appointed IT guy at a business with 20 pcs, we have 1 sbs 2003 server, a consumer grade soho modem/router and a few network switches making up our lan.

We keep on hitting our monthly ISP quota of 60 gb and I don't think its through typical use. I suspect something rogue in our lan but all pcs have av and aren't reporting any issues.

Is there any software which will let me track down the errant computer and end its downloading days. I've had a recent look around for programs to help me find it and tried the ones already mentiond in related questions on this site but nothing is letting me easily identify which pc is using the quota.

As far as I can tell none of my hardware supports SNMP so the only thing I can think of is trying to slip a smoothwall firewall between the network and the router/modem (but thats a pain because its a whole extra computer I have to find, configure and install.

Many Thanks

If you do have a spare PC smoothwall/ipcop is an excellent solution and the configuration/installation is a 10 minute job. I'd also recommend the advanced proxy add on.

Presuming one of your switches has an unused gig port, you could mirror all traffic to it and plug a machine running ntop http://www.ntop.org/ into it to get very detailed traffic reports. But yes this requires you to find, configure and install a new machine.

the easiest setup in your given situation (no new hardware available) is to configure sbs as a gateway (most new pc's have at least 2 nic's) and then use a traffic monitoring tool

my choice would be a linux gateway, which is more flexible with traffic management