If you want a free Antivirus, I would recommend either Avira or Avast. I use one on desktop and the other on a laptop. Each has slightly different details. On both I use the windows firewall, and with the Avira one have the free version of Malawarebytes and Superantispyware, plus Spywareblaster. With the Avast I use the pro version of Malawarebytes as I found that Avasts "live" web scanner affected iCloud so I use the "live" version of Malawarebytes - not as a/v but as a malware scanner..
Only have one a/v. I think either of the two above has a better reputation than Win Defender. Another to consider is Emisoft, but I have not used it.
Coupled with an occasional scan with ADWCleaner I have had no problems for years.