I use Microsoft Security Essentials and it is free. I use this on my laptop and my desktop. ..................... and occasionally check to see if anything is quarantined or has been detected.
I use the same MSE (Windows Defender) and I run full scans per the recommendation of a reputed tech newsletter that I subscribe to. It does take a few hours and the benefits are worth it! Parallel to this I also use Malwarebytes (paid version) My confidence level in Microsoft products has increased after attending the full-day Microsoft Tech Summit in Chicago, last week.