I hate firewalls. One of the first things I do on any personal Windows I install since Vista is to disable Windows Firewall. Usually, that's all it takes... plus disabling the maintenance center's firewall monitoring so it stops harassing you about the firewall, of course.
So when I noticed my PC's Apache was no longer reachable from other machines and that it would no longer ping, Windows Firewall did not come to my mind as an obvious suspect. Only after I realized that the problem started shortly after I joined the install to the entreprise domain did I start suspecting that some GPO was now forcing the firewall. Of course, I then went to check the firewall's status, using the maintenance center. In order to check its status, I clicked Turn on messages about network Firewall. The maintenance centre then displayed:
In English: "The Windows Firewall is disabled or configured incorrectly."
I was quite sure the firewall wasn't configured incorrectly, since the only configuration I had done was to disable it, so I assumed the firewall was disabled and proceeded to waste at least 10 minutes in further troubleshooting before finally realizing that the damn firewall was actually enabled... despite the button offering me to "Enable now".
In the end, this had nothing to do with Group Policy. The problem is you can't even directly turn off the firewall completely; you have to disable for every network type: private, public and - when you're on a domain - domain networks, which wasn't done on my install. So I clicked Disable Windows Firewall, closed the window, and proceeded to verify that the network was working again - which, of course, was not the case. After trying to reset the network card without success, I went back to the panel to notice that my change hadn't taken effect. Great, so for that specific panel, your changes are discarded without warning unless you select OK.
If your Windows machine's networking stopped working after joining a domain and won't even send ICMP replies, do verify Windows Firewall, and do so by going to the configuration panel and to the Windows Firewall panel. And if you need to disable it, select OK.
After more issues with Windows Firewall, I dedicated it a new post.