I have a XP Pro box that will not accept any incoming connections.
It's not the built-in firewall.
The only thing I can think of it that this box is still part of a domain and domain controller no longer exist. The other computers are not part of the domain.
Any idea what would cause this box to stop accepting connections?
Could be that the antivirus software has its own built-in firewall. Can you ping the box? If you can't, it's almost certain that a firewall is responsible. One other possibility that the PC is not online (wrong IP, bad cable, unplugged at the switch, etc.)
By "it is not the built in firewall" I assume that you mean you have confirmed it is completely turned off. If that is not the case, double-check to make sure that it hasn't reverted to being active because it used to default to on but the policies from the domain controller turnd it off during connection (which will not happen now as the domain controller isn't there).
Not seeing its DC should not otherwise affect incoming connections. You could try dropping it from the domain and using local auth (or joining the domain the other machines are on) to verify this makes no difference.
Also, check to make sure the machine has a correct IP address. It may no it it used to get its address assigned via DHCP from the old DC. Not having a correct address would stop it making outgoing connections to other machines on the subnet too though.
