What operating system are you running as defender is preinstalled on vista and win 7 but has to be installed on xp but aside from that defender works with avast but i dont believe it adds anything to avasts protection levels, its crap to say the least, best program to run with avast is Malwarebytes by a long shot and if you can afford the $25 for a life time license it is well worth it for the realtime protection in the pro version but you can of course just use there free version which you have to update manually and run a scan once a week for safetys sake and peice of mind.
