My Computer Is So Infected with Malware I Can't Do Anything.

I have a desktop with windows 8.1. Some how windows defender is turned off. When I try to turn it on it says something about contacting the admin. I searched EVERY option to get defender turned on. None work.

I used my Mac too download an anti virus program to a usb. And tried to install it on my desktop. NOTHIN DOING! It would not install. This computer is TRASHED!

I tried going into recovery. It says I need to insert the windows disk. Well I don't have the windows disk.

ANY SUGGESTIONS GUYs?