Thank you for your responses. I have been using Windows Defender successfully on 2 Windows 10 computers. Now I'm getting a used Windows 8.1 laptop in a few days and the purpose of my question is to have confidence that WD will run effectivetively as well on the laptop I'll be getting. The laptop is going to need a "tune-up" and when it comes to WD, I'll begin by running a full scan as well as the offline scan: even though they take a while to run, I always make sure security is fine.
--- I do run Malwarebytes free as a 2nd opinion program.