Go powerline (Score 1)

I have the same problem in my house. For us (we're renting and can't make mods to the house) the simplest answer was to use ethernet to powerline bridges. Plug the brick into the wall, plug your ethernet into it, and all the computers on the network are sharing the powerlines at 14 Mb/s.

It's not terribly fast, but it's easy and reliable. All the adapters I've seen come with DES encryption built in, and you'll wanna set that.