Just a guess here

The screen shot of the Cnet Avast download page does not match what I get. I see additional characters after .html. I think you may have been redirected to a bogus site. I suspect some malware BHO could be the underlying cause. You might download some of the often recommended fixes such as malwarebytes, etc. I'd do this from another computer, however. Put the file on a flash drive and install it on your PC that way.