Internet Speed

As an addon to what tfs was saying.. try this :

Watch switch lights, isolate high bandwidth user..
Send a winpopup message box to user like this
net send <ip> "Resume not supported"

Then pull the offending LAN cable out of the switch

If you hear a scream from far away.. you've got your man !

I'll try to answer your question ...but first let me draw a picture of how your network looks like ......your machines are connected via hubs or switches to a backbone switch which in turn gets connected to the router . You say that you connect to the internet via a proxy present at your ISP's premises , so its quite possible that even your WAN interfaces have a private IP .....i may be grossly wrong in figuring it out, do correct me if i am........

the best bandwidth monitoring software that i have come across is MRTG .you can install it on your router as well as switch .It can generate daily , weekly .....and so on html reports about the bandwidth being consumed ......shows both upstream and downstream traffic ......run it on your WAn interface and start downloading a big file ...your downstream traffic should be in the region of bout 100Kbs or so ...bu this again depends upon the type of service uve taken from your ISP and the policies of your ISp as to wether any bursts are allowed or not .

With this u can monitor if any single machine is generating excessive upstream traffic and congesting your network...hope this helps