tracking down a trojan?

[Biggerhammer]

I just took a job as sysadmin for a mid-sized comapny. Our mailserver cheerfully tells me that we've sent thousands of phishing messages for paypall. The mailserver is basically useless for tracking down which of my machines is trojanned. short of walking around to every machine and running netstat, how do I track down which machine(s) have a trojan?

Thank you.