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How do we determine what ports are needed open? How do we close other ports? Thanks for the info and your help!
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A good place to start is
Shields Up! This tests your current set up, tells you what ports are open, and offers some help as to what they're for. In short, you're best off having all ports closed by default, and only opening ones that specific applications need (applications which do require certain ports should say so somewhere in their documentation). What you're trying to prevent is a worm seeking open ports to infiltrate through, and Shields Up! will tell you how vulnerable you are.
As to how to close them, you'll need to see the documentation for your router, if you have one. If you don't use a router, a firewall will serve the same purpose (I use both); as I said, I use ZoneAlarm, but you could use Norton or the new Windows XP firewall (I tried the XP one a few months ago, but it was pretty buggy then, and I switched back to ZA).
There is a free version of ZoneAlarm, though I think the additional features offered by the commercial versions are worth it. The nice thing about it is, if something new happens (an application asks for Internet access, etc.), ZA will pop up a dialog box asking whether to allow or deny access, and give you the option of having it do the same thing every time the app repeats it. So, the first time you use IE, for example, you'll need to tell ZA to allow it, but if you check the box, it will remember that and not ask you again. Check out the
Zone Labs web site for more info.
-Mike