John is trying to run Windows Defender and it's been turned off in Windows 8.1. How can he turn it on? Leo suspects that since John is running Malware Bytes as well, that it is blocking Defender from being turned on. Leo says first, you don't want to run two different antivirus software. Often they see each other as a virus and frankly, they fight each other. So make sure you only run one. Also, Microsoft's own AVS is adequate to do the job. The real key to defend your computer is to modify your behavior. Always keep your OS up to date, and avoid going online where malware lurks or installing unfamiliar apps.