There are three machines involves: 1xWindows 7 Professional, 2xWindows 7 Home Premium. All are in the same workgroup on the same physical LAN segment. 1 7HP machine can't be seen by either the 7P or the other 7HP machine, but can see both of the others. Sharing and discover are both on, and the firewall is configured correctly (and when turned off it still doesn't show up).