Users Under My LAN

Dear Friends
I Have in my company LAN sever with Win2000 and the clients are Windows XP . one day my Internet connection become verey heavy and slow when i check with my ISP they tell me that one of my workstation sending backets on huge ammount . my quastion what is the most powerful utility i can use to check and descover which machine sending that backets and how could i use it ( Notice : i need trusted application ) and i have a network with 9 servers the primary one with 2000 Server . and i have about 70 clinet )
please inform me about that .
Thanks
Yours
Emad Mishry
mishry@kmefic.com.kw