Isn't Microsoft Windows Defender an anti-virus program? Malwarebytes is a malware block and cleaner. You need both types of defense on your computer. If Malwarebytes does in fact look for and clean viruses too, then, maybe not -- in that case there is a logic for having as many different virus scanners on your computer as possible to back each other up. However, conventional wisdom is that two or more operating virus scanners often conflict and create issues and are not worth the any overlapping gain in net security.