somewhere in your house will be your main socket, with a detachable front plate on it. . . take it off and there is a test socket, plug your router into that and log onto the router homepage to see what speed your router see's. . . if its low then there is a problem outside your house and your hands, either a problem with the lines too your house or at the exchange . . . if its fast then there's a problem in your house somewhere, maybe a knackered ADSL filter or faulty socket etc etc.

cheers
Tim