Active Ports - easy to use tool for Windows NT/2000/XP that enables you to monitor all open TCP/IP and UDP ports on the local computer. Active Ports maps ports to the owning application so you can watch which process has opened which port. It also displays a local and remote IP address for each connection and allows you to terminate the owning process. Active Ports can help you to detect trojans and other malicious programs.
More info:
hxxp://www.microsoft.com/windows/catalog2/default.aspx?subid=22&xslt=detail&pgn=21b41781-45df-1b58-b64c-40ef15302050

Very useful. Checking it out... :-)