Network Monitoring Tools

I was wondering if anyone here knew of any quality networking monitoring tools so that my company could find out how much bandwidth each computer is using in a day,and use it to see what bandwidth they are using on the network. I believe that at least one of the computers, possibly more has a bad NIC card and is continuously sending packets and flooding the network.

Any programs out there that could do this? Our router currently doesn’t use SNMP, so I have had some problems finding a decent utility.