There are a number of free antivirus programs. Panda Antivirus works well in my experience. I like that in the settings you can turn off "Panda News" and "show relevant security news" and then it runs quietly. It doesn't nag you to upgrade to the paid version either. I used to use Avast and AVG but they got annoying. But you can test those options out too. Panda does ask for your email...but you can x-out and ignore it....it still works and updates and does everything.
The difference between the free options and the paid options is that the free options tend to be just antivirus and nothing else (I like that). When you get into the paid options they start to bundle other things like, spam protection, and browser search protection, and firewalls, shopping protection, password vaults...etc. They try to add value for being a paid product. If you need these other things and you like a combined solution than the paid approach may appeal. But sometimes offering all these added functions leads to bloated software that slows down your computer. Some of the extra features are unnecessary, for example spam protection....you have that already with the free email services, firewall that is built into Windows...may not offer the level of reporting but its there, and other features web filters are becoming less relevant as browsers are starting to block some known bad sites and a lot of home routers have built in filtering options (like Asus has TrendMicro malware blocking built in).
Windows Defender is built in and takes over if your installed antivirus gets disabled.
I like running the free version of malwarebytes as a periodic cliean-up in case something got through in combination with Panda. But the paid version offers live protection.