How to find rogue spammer on our network?

The phone company keeps calling us that there is a rogue spamming computer on our network. We have nearly 100 computers on our network. The phone company cannot tell us the ip of the rogue machine because all they see is the ip of the switch that connects us to their modem. The did tell us that it looks like the suspect machine has a bot on it sending spam mail through SNMP. How can we determine which machine it is? We use static IP addresses here. Thanks!