Malwarebytes is not a full-fledged anti-virus.
You don't generally get "visitations" from a virus online unless you're visiting somewhere or downloading something that is risky. They can also come through email attachments, flash drives, etc.
However, people sometimes inadvertently open things they shouldn't or visit a bad site by mistyping the URL or this or that. It happens. If there isn't anything important on the computer, you might be ok. I still wouldn't recommend it.
Surfing the net without AV or Firewall is kind of like tossing your parachute out of the plane first, then trying to catch up to it as you fall. While it might be a cool stunt, done enough times will likely end up in you not catching the parachute.
I'd bet that if you installed a full AV program (like Avast!) and ran a full scan of your system, it will find threats that MBAM missed.
It doesn't matter what version of Windows you're running, running without AV, infections are a certainty.
