I have a user who's writing a network server program, but his computer isn't accepting incoming connections. The program (as well as other network services, like a VNC server) works fine when initiating the connection from localhost, but not from any other computer on the network, including pings. Outgoing network connections work fine, including, to be pedantic, the replies, so it is accepting incoming data, just not unsolicited incoming data. I've turned off both the Windows firewall and the Symantec firewall that was installed on the computer with no change in symptoms (well, except that the Symantec firewall also prevents connections from localhost).

I've upgraded the drivers for the network cards. I've tested it with both the wired ethernet interface and the wireless interface with the same results. I've installed Wireshark (formerly Ethereal) and it doesn't even see the incoming packets, but it does see other packets, so Wireshark is working properly. When initiating a connection from a remote computer, the remote computer does get an arp reply, although I haven't made sure that it's receiving it from the computer in question directly.

The computer is connected to the same switch as all of the other computers that are working properly. I have tried it in different ports on that switch with no change in symptoms.

Does anyone have any idea what might be going on?