How to stop Spam sending Trojan Viruses

Hi All,

Recently, one of the computers in our network was affected by a Trojan virus which is believed to have sent thousands of SPAM messages.

Due to this, our domain IP has been blacklisted and emails from our domain aren’t getting delivered to clients.

We have ultimately formatted all the drives and reinstalled the OS in that computer.

But, how do such Trojan viruses intrude when we have a popular anti-virus program in installed and updated every day.

Please share your thoughts on how to prevent such incidents rather than fixing them.

Thank you.

Best Regards,
Aravind.