browser hijack

So I've got a couple of problems here that I can't seem to solve or find anyone who has an actual solution to them. Firstly, there's the yahoo redirect when I search for something through my default search engine, which is google. I had this issue once before, which was about a few months...

Am not a computer expert myself but have been able to parse threads and do the needful to keep my pc working and effective, until this time.. . The relevant history.. Win 8.1 Factory fitted (No Backup CD/DVD ever made) :-( LENOVO G50-30 80G0 64 Bit, Anti Virus used Avast (Now uninstalled)...