When you are looking for a free anti-virus app, make sure you choose one that is lightweight and actually protects you.  I have discovered that the default Windows Defender (Microsoft Security Essentials) usually pops up after Malwarebytes catches something.  If other apps are catching things before it does, that goes to show a lot about how effective the Windows Defender is.  Malwarebytes is great for protecting against Malware, but you will still need a regular Anti-Virus program to to get better defense.  Malware makers are always looking for new ways to exploit Windows and get into your computer.