If Norton is blocking LW, then I'd reckon closing LW completely (i.e. also in the system tray) and then deleting all references to LW in Norton. Then close Norton and reopen it. Usually it should then ask you about LW again.
But before doing so, since you said that you have a DSL modem and not a router, enable UPnP in LW, then follow the steps I've just recommended.
