Some problems in windows 8

1. I have tried to activate the 'windows defender'. I select auto, but it goes back to manual mode. When I click 'start' windows to tries to start it, but reports that it has stopped.

I have 'AVG" antivirus free addition. (I had uninstalled Mcaffee as it had become a nuisance, and I was repeatedly getting messages to purchase it. and I installed AVG)

Does AVG stop windows defender from running? If so, is it better to uninstall AVG, and have only windows defender protect the system from viruses ?

Thanks,
Preet