ejd1:
Native tool in Win NT/2K/XP = NETSTAT. Is a commandline tool. Run the following syntax: "netstat -an" (-ao for XP). It will show the TCP & UDP ports your computer is either listening on or established. Listening means there is a program of some sort (or OS) listening for something to talk to it at that port number. Established means what it says: a connection has been established.
Freeware and commercial tools that I recommend are:
* DiamondCS OpenPorts (http://www.diamondcs.com.au/openports/)
* Foundstone's FPort (http://www.foundstone.com - go to Resources | Free tools section)
There are many ports that are native to the OS and shouldn't worry about but if you want you can check the following web site to see what uses a certain port:
http://www.treachery.net/security_tools/ports/
There are plenty of sites but I use that one the most.
There's a lot more details to this and sorry I cant post more at this time but this is a start. Also, search AO here on "ports" or something like that and I'm sure you'll find many valuable postings on the topic.
/edit
Forgot to address the closing the ports question: you need to know what is listening on a certain port number and then end that task or process.
/edit
Good luck.
