One Computer On Network Can't Be Accessed By The Others

I've got 3 identical PC's that were bought and configured at the same time, all running Windows 7 off a Netgear router. Though all 3 machines can successfully ping each other, one machine can't be accessed by the other two through windows explorer meaning it can't participate in LAN games or share files with the other 2 machines.

I also seem unable to register my copy of Starcraft II because it doesn't recognise any return data from the Starcraft servers, though it works fine on another machine (not sure if this is related). I've disabled the firewall on the unaccessible machine and believe file sharing is activated but it hasn't worked.