Microsoft has improved the flakey Windows networking model in Windows 7 but it’s still not perfect. The Homegroup system should make networking  a breeze, as long as all the computers involved are running Windows 7. However there are some quirks. One such annoyance crops up when using security apps which include their own firewall. In order for the Homegroup discovery service to operate Windows Firewall must be running and so if your security app has its own then it is likely to have turned the Windows one off thus making it impossible for your computers to find the Homegroup you’ve just set up. Something to check when you’ve been banging your head against this particular brick wall.