I have a virus on my laptop but virus scanners are not picking it up?

I know i have a virus on my laptop. It is running really slow, i can't watch videos or live feeds, random windows keep popping up with advertising things on it. When I try to search for anything a detection warning from Antivir guard comes up saying C:/users/melanie/appdata/local/microsoft… that contains recognition pattern of the html/infected.webpage.gent html script virus. I have run four different virus scanners; trendmicro housecall, antivir, mcafee, and malware bytes and they all say that there is no threat!! What is this virus? How can I get rid of it