What is the quality of the picture on your living room TV? If it is ok, buy a two-outlet signal amplifier around 6db gain. Feed the main aerial to the input of the amplifier. Take one output to the living room set. Take the other to your Media Centre.
If you need a long run of cable to your setup, make sure it is low loss 75 Ohms. CT100 should be ok. Your friendly TV dealer or electronics shop should be able to supply your requirements.