Internet Security

I am working on a small peer to peer network (4 computers with Windows 2000 Pro). The network has been running fine, but when I installed Norton Internet Security or McAfee Internet Suite, the network breaks down and there is no communication. They can all still get on the internet, but I have no network. When I uninstall the internet security, the network returns without any adjustments at all.
It seems I can have a network or I can have security, but not both.
Any suggestions?