I don't know what to suggest... Maybe just check the network status on every PC (Ctrl Alt Del, "Networking" tab?) and add them up.
Also if you feel it's 1/2 as fast, call your ISP and ask them about their service. See if they have been making any changes and ask them if there's anything they can do to boost the signal. If you are that secluded and have DSL chances are you're pretty far from the central office and won't be able to get the most from DSL.