I have been told that Windows Defender will activate IF you don't have another anti-virus program. I had Norton which came with my puter but got it out of there quick because it was causing a lot of problems. I downloaded AVG free one and have used it for years. Windows Defender will not activate if you have another.
On my Vista machine I had AVG, SpyBot Search and Destroy, AND Malware Bytes, but have been told not to have more than one of Win 8.1. I do have CCleaner which always finds issues.
AVG has alerted me many times about threats.