1. If you are going to use your dial up until the line is ready don't olug your broadband modem in until you are ready to install
2. To make it work just on the point where he PC connects - you may need to fit a filter to the phone downstairs as well to get rid of noise
3 Filter plugs in directly to the modem - use your extension lead and connect the filter to the end of it
Is your broadband modem USB or ethernet i.e does it have an ethernet socket on the back as well as a USB?