How do i get rid of antivirus virus.?

I have Windows Defender and a antivirus on my computer and this other antivirus just "popped up" on my computer. It has blocked my windows defender and it keeps popping up telling me I better buy their service. I know better than to get suckered into this thing. I want it gone. It even makes a popping noise when it pops on my screen. I need help. They are calling themselves MS Removal Tool Firewall Alert.