Unknown traffic coming from my server!

I’m running a mission-critical Windows 2000 Server with SQL and web services. For the last week it’s been generating 800K per second traffic in and out. I’ve replaced the NIC, to insure that it’s not a freaked out hardware problem. I’ve run anti-virus scans on disks, memory and in safe mode, but NO change. I do not see any unusual usage in processes under task manager. PLEASE, can someone help give me a step-by-step method of finding what’s generating this! HELP!