Windows: In most situations, IM apps like Hangouts will do fine for your instant messaging needs. If you need something more local, LAN Messenger can help.
Since LAN Messenger works over a local network, it won't break down if you lose internet access. Additionally, it automatically detects any other users on the network as soon as they install the app. For multi-storey homes or small business and offices, it's a capable backup with minimal setup.