Check Open Port (Windows XP)

If you need to find out what ports are opened, established and closed, use these commands in the command prompt.
type: netstat -a or this one: netstat -an, this shows the list ports that you use in your computer.

type : netstat -an |find /i "listening" and netstat -an | find /i "established", to know who is listening or established.

type : netstat -an |find /i "listening" > c:\openports.txt, to save the ports list to a text file.

To get a list of all the open, established, closing and other used ports: netstat -ao

Please see the list of common Trojan port numbers from http://www.sans.org/resources/idfaq/oddports.php