For Windows computers, I would just retire/upgrade anything not running Windows 8.1 or Windows 10.
If you're on Windows 8, check the Windows Store for an update to Windows 8.1.
If you're on Windows 10, make sure you're on a version that is supported. We can get into how if needed.
Everything else is out of support and not getting patches. That includes Windows Vista, 7, XP, Windows for Workgroups 3.11. Microsoft won't fix security problems.
If you have the option, consider staying in Windoww S Mode. It's like traction control - there are solid, sane arguments for turning it off, but it's there for a reason. It does leave you browsing on Microsoft Edge, but Edge is a really good browser now. It even runs Chrome extensions from the Chrome store.
I advise using the built-in AV. I don't think you need any third-party product. Your security budget, both in terms of money and time, at home is usually better allocated to password management, multifactor authentication, and software patching.
There's a good chance that not every user in the house needs to be an administrator of your devices. Consider using the control panel and making admin something only grownups have, or better, a separate account you only use for computer maintenance... Just don't lose the password. Maybe put it in a password manager?
If your household has Apple devices, keep them on the latest version of iOS, iPadOS or the last two versions of MacOS. If your device doesn't support the update, it's time to start shopping for replacement devices.
If you have Android devices, check for updates in the settings app. If, after all updates are done, your patch level isn't a date in the past six months, it's probably time to start shopping for a new device because your manufacturer either doesn't care about the security of their customers or because your device is just old.
If you have Chromebooks, check your device against Google's Auto Update Policy page to make sure your device's last update isn't on some date in the past. The AUP page doesn't show an end date in the future, it's time to start shopping for a replacement.