To the OP, what kind of heating system is in your house? If it's forced air heating, you can use the cold air return ducts to run cables through the floors, and then just under the baseboards along the floors. If the floors aren't carpeted, then it's a bit more challenging, but you can rip off the baseboards then nail them back down, which will require re-caulking and maybe some painting. But it's not impossible to run network cabling through an older house, depending on your determination. Don't forget the "golden rule" of running CAT-5e, no length longer than 100 meters (or 328 feet).