Sounds to me like one of your computers is connected to the net and acting as a server to your other computer.
Perhapse if you were to get a router things would work out more even, often routers will share the bandwith evenly.
Few people need to use 80% of a broadband connection, Perhapse there is spyware or addware (Dodgy software) That is using up a lot of the bandwith.
To check for spyware i use Microsoft anti-spyware - http://www.microsoft.com/athome/secu...e/default.mspx
Routers can be purchased from most electronic retailers - i brought my one from e buyer - http://www.ebuyer.com