Hi again, silverfox. You haven't yet had an answer to your question regarding paid for programs.
It can often come down to a personal choice. But there have to be considerations for resource usage and ease of use. Your best bet is to browse for articles on the subject, study them and decide. One point. The paid versions include, as a rule of thumb, xtras such as Firewalls and other goodies. But keep in mind that paid or free, each program will be using the same basic antivirus scan mechanism, and from, the same data base for that particular organisation.
So if you feel the need for those "xtras", then, do browse and decide. Only because it was free through my resources, I used Kaspersky for a while. It proved to be quite heavy on resource usage but apparently did the job. Oddly, when I removed it and put Defender back on, Defender found a Trojan on my computer!
Avira, I used from XP days and into Vista. Also a good paid for program.
Have a look here for price, facility and performance comparison.
Best Antivirus Software for Windows 8 2013 | Windows 8 Antivirus Software Comparisons - TopTenREVIEWS P.S. fwiw. I have had no hangups with Defender, and I receive notification of definition updates daily.