yahoo taking over my mac

Hello all i know this problem has been answered many times but i figure i need an update

i have a Mac

and browsers i used are Safari and Chrome, and even when i do searches in google i get sent to yahoo

which keeps popping up and is quite annoying

i done the suggested Extensions thing and still does it

i ran 2 virus scanners Bitdfender and Protectworks, they did pick up some corruption but didnt fix problem,

i must have some type of malware or ad ware some where

suggestions please? if your directing me to a program to use, make sure its not a virus filled one please

all the help i can get on this please

look to your kind responses!!!