I am a network admin novice that inherited an existing Server 2003 Standard R2 network. I recently noticed that the server has windows firewall installed, but disabled. When I enabled it, none of the connected machines can access the internet. The tutorials I have read talk about adding ports to the exception list. For example adding port 80 for web access. But this does not help my situation. I would be grateful if someone can point me in the right direction in solving this.

It's possible that the previous administrator installed a different firewall or is using a router with firewall capabilities and that's why Windows Firewall is disabled.

I thought of that, but could not identify other firewalls running. No router. Thanks for the response though.

Does "no router" mean that there is no router with a firewall? There has to be a router.

Yes. What I meant was that there was no firewall on the router. Anyway, I have resolved my problem by unblocking DNS port UDP 53.

I'm glad you solved the problem.