The following is an extract from Windows help & support in Vista, but dont know if it applies to W7 but its worth checking out. Also be aware that XP does not support network discovery but there is a hotfix available from Microsoft, I have tried it and it works ok. Any of your PCs that have XP will need to have the hotfix installed in order to participate in network discovery.
Hope this is of some help.
Network discovery requires that the dnscache, fdrespub, ssdpsrv, and upnphost services are started, that the Windows Firewall exception for network discovery is enabled, and that other firewalls are not interfering with network discovery. If some but not all of these are true, the network discovery state will be shown as Custom.