Windows Defender not working due to malware

Thanks. After I posted my message, I actually did some research elsewhere to figure out that Avast disables Windows Defender. The way it was doing it was puzzling me, but, now that I know, that just means Windows Defender isn't a problem. But, my computer is acting weird in other ways. When I plug in a second monitor or change resolutions up to or down from 4K, the screen flickers for about a minute or two, rendering it inoperative during that time. I've gone into Avast to do scans of the entire computer and all connected drives with sensitivity set to high a few times, and I've done boot-time scans on high sensitivity a few times, also. My Avast logged a quarantine of a threat called "VBS:Gamaredon-CM [Apt]" in the location C:\Users\arthu\AppData\Roaming\Mozilla\Firefox\Profiles\rw2jz9wz.default-release\prefs.js on March 22, which may be the time the problem started happening, but I'm not sure, and the error message I get that I cited in my original message has happened a few times since that date. I normally use Firefox. I don't know what my computer is doing trying to access a black website unprompted, but I've gotten the Avast message preventing the connection at least twice. Any thoughts?