Spam

I has been receiving complains that my server is sending spam. I am pretty sure that no one of my users is sending this kind of messages, so probably one of my PCs has some worm which is sending it. I verify one of these messages, and they are really spreading from my IP.

I has Corporate TrendMicro antivirus (supposedly, the most reliable), so I am really lost of where can I start searching the solution for this problem.

Any thoughts?