VIRUS ATTACK ON COMPUTER THROUGH AVAST! IMITATOR

My PC has been attacked through a download from what looked to be an avast! support pop-up which offered to scan and clean up items which were causing the computer to run slow. What a disaster - and now I can't get rid of them.

This is what they do:

Where on earth does yahoo come from? I don't use yahoo, and don't have any use for it. I don't use google chrome although it is on my pc, but I do use firefox, and I have tried uninstalling it thinking the virus attackes would go, but when I re-install firefox, the pests return!

To try and get help from any of the major browsers is like asking the Palestinians and Israelis to combine and live peacefully as one nation, so there I am stuck with these attacks on my pc and my only escape is to stop using microsoft altogether and just use linux which is not subject to such annoyances.