Internet Usage Monitoring tool

Peter there is the old cmd netstat works on windows 7
 simply open your run from start orb then type in cmd press enter
in the dos box where you see the curser type in netstat and press enter
it will list all established network connections. This will show you who/or which internet is connected. Here you can see I have my ISP Bigpond as well EE on internet as well as the other computer here on my lan called Stuart PC, I'm User PC.
It won't show you how much is being downloaded but finding who is active helps a lot. Quick access as well

noone knows what type of router you have so some of the solutions may not apply - get onto Wireshark formerly known as ethereal (make sure to press yes on winpcap when installing) and see which IP address is sending all of the requests.

After that perform an nslookup and you then know the computer name.

Most of the time its torrents.

In the future perhaps setup a web proxy
or even use Open DNS

Squid is free and open source

GFI LAN proxy is a paid one - there are many paid others
Windows included ISA in server 2003 sbs. (i think)