Bandwidth usage monitoring tool

[Yu Meng Chong]

Hi all, 

Not really a Linux question, but I am hoping that someone can help me with this: I need a software (or appliance) that can help me to monitor bandwidth usage for each node on a network. 

The reason why I need this is: a friend's network keeps getting infected, periodically, by worms and malware from China, and Symantec and McAfee don't seem to be able to detect these things (I've often wondered if this is because they can't read Chinese). Of course switching them to Linux or even Apple would help, but then I wouldn't have that guy as a friend anymore.

Anyway, in other situations, I have found that the only way to detect which PCs are infected by China viruses is by looking at the bandwidth usage of each node on the network. Sonicwall has an integrated bandwidth monitor that shows how much traffic comes out/goes into each IP address, and I am looking for something similar. My friend's network is a switched network (unmanaged, so no web interface there), and both router and firewall are the cheap kind with no bandwidth monitoring features. 

Thanks in advance for any suggestions!

pascal chong