Minecraft Java/LAN problem?

I have 3 computers on a LAN at home. All 3 have minecraft installed as the family love to play together. Recently I upgraded one PC from an AMD dual-core to an Intel i7, and now I’m in network hell, in that 2 computers can join the same game but a 3rd PC won’t.

All three are running JDK7u71, all 3 are getting the IP’s from a modem router running DHCP. No mac address filters etc

All three can login into minecraft & play single-player Ok.

All three have windows firewall entries for both the JDK and for minecraft (JDK & minecraft installs did these on their own it seems).

All three are running the same version of Minecraft/Forge and misc mods.

2 PC’s are running Windows 7 Ultimate.
1 PC is running Windows 8.1. <— This is the one that cannot connect to a LAN game with the others.

Any ideas as to what else I should try?