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You might want to look into something like NTop which reads the traffic from your network and can tell you precisely how much traffic is being sent from each PC but also what the traffic is too. The main problem with NTop (and all other packet analysis tools) is getting visibility of the traffic in a switched network. There are a number of techniques that can help... Practical Packet Analysis by Chris Sanders documents a number of them. Using a network tap can help a lot too.