Assuming that they are both connected to the same router the only thing that could be blocking them is something interfering with their respective netbios information. Computer name conflicts is common, disparate work group names, improperly configured subnets, etc. But the most likely cause is third party software, firewalls and or internet security suites that act like firewalls. As I said earlier you need to uninstall them. If you can ping them by ip address but not by name then something is blocking netbios, or the netbios cache is corrupt or non existent. If the windows 7 firewall was tinkered with at all, then turn it off also until you get your network issues sorted out.