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Did you also find the Avast antivirus program installed, or was it just the browser extension? And did you already have some active antivirus on your system, with real-time protection enabled?
Avast antivirus was installed as well. I had to remove it from "Add and Remove Programs" and restart.
I don't have another antivirus other than the default Windows Defender that is a part of Windows 10. I'm not sure it was enabled though.
Well with Windows 10, Defender will fully activate itself unless you disable it, or if it automatically detects a 3rd party antivirus solution installed. If it detects another AV, it will go into passive mode. If you uninstall your current AV, Defender will pick up the slack. That's actually a good call on Microsoft's part!
But the installer for Avast had to arrive on your machine bundled with something else. I can't imagine it just appearing out of the ether. In my case earlier, I had Bitdefender running on a Windows 7 machine, and when I updated CCleaner, it installed Avast! There was a known issue where that install was supposed to be optional, but apparently CCleaner missed something in the GUI that did not show ME the opt out. It was probably a rare corner case, but it did happen!
That was long winded, but I must assume something else was updated on your computer that invited Avast aboard.