MICROSOFT ******* SECURITY ESSENTIALS. Personally, I don't care for any of the free anti-virus solutions that I've found. Sure, avg, avast, avira, etc. all work, but they hog resources, slow down your computer, and just seem to be a mess to configure and operate. MSE is free, and it was built by microsoft, who know more about the security flaws in windows than any third party company.
That being said, I also like malwarebytes, though it is just a simple run/scan program, it's not constant active protection. It does an incredibly good job though, and you can simply run it now and again to make sure your system is clean. Also I haven't looked into it but I'm sure there is a way to run scheduled scans with Malwarebytes.
So go with MSE for active protection, unless your copy of windows isn't validated. Then you'r outta luck, pal.