Works fine in one room, but the Macbook Pro he set up in the kitchen is constantly buffering. He’s using a 50-foot ethernet cable. That could be one problem (although CAT-5 should go much longer than that). Maybe it’s the way Apple is handling the streaming Windows data?
If you can run a wireless network instead of hard-wired, and use either 802.11g or 802.11n, your internet speed should increase. If you use ethernet cable, the signal strength will degrade over long distances.
A possibility may also be viruses or spyware on the network, but the speed of the network is more likely the issue.
You might want to look at an Internet radio – C Crane sells a great one.