How do I close an open port?

Question

Some guy called from our ISP today and said that a hacker hacked into an open port from one of our two computers and is sending nasty e-mails. We checked all our e-mail addresses and saw nothing in the sent folder that we didn't send, but that could've been deleted.One of the computers is an old dell and another, the one im using now is an HP Laptop. Anway, we bought AVG security for my laptop since we wanted a better one than Spy Sweeper and did a scan. Nothing but cookies turned up which is good. We don't know which computer is being hacked and we also don't know how to close the port. So all I really need to know is how do I close that open port?! Thanks to all who answer.

Answer 1

Do you have a corporate firewall?  If so, block TCP port 25 outbound and log the drops, this way you'll identify the offending host(s).   If you don't need port 25 outbound at all, just block if for everyone.