Bandwith monitoring/Management

I have a serverless workgroup of 20+ PCs (all windows XP) connected to a router, I'd like to be able to impliment some bandwith monitoring to be able to root out people that exploit bandwith through downloading and such. I imagine I'll need to setup a PC as a gateway for any of the bandwith monitoring tools to work ? However I would like to keep using windows xp instead of the server editions, so what are my options ? How do I set it up?

 

Try Untangle. Works great and is free. There are also paid apps you can download too.
http://www.untangle.com/