Malwarebytes excellent when you need it a resource drain when its not needed.

Usability-wise, it's pretty good, and it gets the job done. But once that's finished, the nags, the pop-ups, and the fact that it slows older systems down recklessly really cost it rating points. It becomes a clutter, and one of the first things we check when we receive reports that a PC is slow is whether it's running malware. Once we uninstall it, the PC is usually easily 40-50% faster. That's too much in the way of resources for something that wants to always run in the background.

Now, I gave it that rating because it's a handy tool for diagnosing issues. Quarantining them, and most of the time, it does fix the problem. Though with rootkits, it's been hit or miss, and sometimes perfectly valid software gets flagged erroneously. However, once you've run it, it tends to run continuously, consuming far too many resources and being a real pain to uninstall, sometimes even causing issues.