Why Is The Windows Firewall Blocking File Sharing

I used the standard trick of assigning permissions to the "Everyone" group to allow file sharing among all the Windows 7 and winXP computers on my home network. All was working fine until my son took his computer to school and they made him load their security software. When he came home for the summer, his computer could still see the network and file shares, but none of the other computers could see his shared files. This persisted even after I uninstalled the anti-virus software they made him install. I can fix the problem by disabling the windows firewall on his computer. That is probably safe here because of the firewall in my router, but I'm worried that might not be safe when he connects to other networks. Plus I can leave the windows firewall enabled on all my other computers, so I suspect there should be a better way around this problem. If I click "Diagnose" when my computer can't access my son's computer I get a message that says that windows can't find IPC$. So on my son's computer I opened a dos window and typed "net share ipc$" (a command I found from a google search about this problem). The response to this command was "System error 5 has occured; Access Denied".