Ed,
"
Windows Firewall in Windows XP SP2 is globally enabled by default. This means that, by default, all the connections of a computer running Windows XP with SP2 have Windows Firewall enabled, including LAN (wired and wireless), dial-up, and virtual private network (VPN) connections. New connections also have Windows Firewall enabled by default. "
That being said, you may want to take a look
here and see if this helps you get your connections back.
I am only up to SP-1 on this machine. I tried updatiing to SP-2 a couple of weeks ago and it didn't like it, so I rolled it back to SP-1. I have SP-2 on a box at home so I'll see if I can make some time to have a look at it and post some more info then. As far as I recall though, I believe it may be as simple as disabling the firewall, making all your connections, and then re-enabling the firewall again. I believe it remembers your connections and their settings. Of course, I could be wrong though...
Randy
edit: Ed, btw, you haven't said whether or not you're using a router or just ICS through a hub, connecting all your pc's.