Network internet traffic activity monitor

Hi Guys,
I work in an office with 20 computers on the network and was wondering if there are any cheap/free internet traffic monitoring tools that you would recommend that could be used on a lan. Its just to monitor any pc's on the network inadvertently making big downloads/uploads that affects the internet speed. It may be handy for security too in case any programs are auto updating or email auto archiving. The monitoring tool could be installed on the windows server 2003 itself or on one of the network pcs.

Thanks

You can try Netmon.

You can try GFI WebMonitor. It uses multiple antivirus engines.