Microsoft Port Reporter
Microsoft has release a handy utility, "Port Reporter", to log which ports are being access on a given computer. It runs as a service and the logs are in a comma delimited format text file. I'm currently using this on my dev box and it's pretty interesting to see which applications are accessing network resources and on what port. The most interesting part is to see how often a given application goes over the network for something.
Here is a pointer the Microsoft KB Article that discusses how to download, install, and run the utility:
http://support.microsoft.com/default.aspx?scid=kb;en-us;837243
This utility can run on Windows XP, 2000, and 2003.
BTW, don't forget to install the Port Reporter Parsing utility (also found on the above URL). It makes sifting through all of the port data a bit easier. BTW, watch out for the log file to go crazy on front end production servers. Don't know if it's an issue, but something to keep mindful of.