...what I finally did was disconnect the network. (I've found that playing the Halo Demo online through the router causes so much lag it's virtually unplayable for me and others).

Then, I shutdown Zone Alarm so others could join. They still complain of lag, though...

1) Do you have a firewall?
2) Are you running the Windows Firewall that came with Windows XP SP2?

Check those.

Good luck,

Graham