Would you say Windows Defender is as good as, or maybe even a better choice than Avast Free?

Just installed Ad Blocker Plus, working fine.

Yes, but mostly because Defender is built-in and doesn't require the use of any tricks that other AV's use to inject themselves into the low levels of the operating system they need access to in order to do their job. Those same tricks can sometimes be exploited by malware to give the malware the same low level access. That's something you don't want to happen. Finally, now that Defender can be configured to run in a so-called "sandbox" (a software construct that prevents any malicious code that might get loose inside a process from having any access to things outside that process) you're going to be about as safe as you can realistically be. The sandboxing capability is relatively new. I don't know of any third party antivirus that offers it. Here's an article describing it in more detail:

Microsoft Sandboxes Windows Defender

You definitely only want one anti-virus. Here's a quick explanation why.
https://www.kaspersky.com/resource-c...virus-products

Thanks. :ho: