Check the TNSNAMES.ORA file for the HOST= entry. In this file, I found that the machine name was set for UnitedWay-4. I checked Control Panel-System and found that the machine had been given a new name...and this was the reason Oracle was refusing connections ('target host...does not exist' makes sense, eh?) and the listener refused to start. I changed all instances of UnitedWay-4 to 127.0.0.1 in TNSNAMES.ORA and LISTENER.ORA and this fixed the problem.