Is your Malwarebytes a free edition ?
A free edition is an "on-demand" protection program. Meaning it does not run until you click it open, and it stops the moment it finishes scanning.
If that is the case, I would suggest that you install a 3rd party program to help Windows Defender.
However, if your Malwarebytes is a paid version, then it is a "real time" protection program. Meaning it starts to work the moment you turn on the computer.
If that us the case, I think you are a-ok. Some might suggest something else to supplement. It's your choice then.
Personally, I have the free Malwarebytes edition. I have Avast free edition as my "real time " protection program.
I have Windows 8.1.1 Pro.