Hi, all of a sudden Yahoo and Google did not work on Internet Explorer and Chrome anymore (“Page cannot be displayed”), at the same time all other website that I regularly use, worked fine. I have tried all sorts of things to fix the problem, but did not succeed so decided to download Avast. Unfortunately, Avast has not improved the situation. I have now downloaded a different browser, called Opera, on which I can access Yahoo and Google.
Does anyone have an idea, what the problem is and how to fix it? It seems to be related to the search website only. Bing, however, was not an issue.
Thanks,
Matthias
Welcome to the forum. 
What other security programs are installed ???
Any spam or Ad blockers ???
Could be a browser infection.
If you want us to check things, follow the instructions:
https://forum.avast.com/index.php?topic=53253.0