need to monitor which users are using most internet bandwidth

on our network users are running kazaa and other file sharing crap and its causing the internet which is 1mb dedicated leased line to be real slow so we need to monitor the internet usage but we only have mrtg which shows the entire company's upload and download stats in graph format. this isnt enough so suggestions for other tools on win platform are welcome. this will be run on a win xp box on one single subnet 192.168.0.0 network with about 90 hosts.
Anand (MCSA - self study route)

 

Network Probe.
http://www.majorgeeks.com/download1089.html

 

May be this can Help: Can I Monitor my LAN, control the usage of the Internet, and Network Resources by other computers from my main computer?