Windows defender (wins8.1) automatically turns off

Hello,I really really need help right now. My pc has been unprotected for days. Mcafee is expired. I'm having issues installing avast premier which i sent onto a flash drive from a friend's computer and its doesn't move beyond the part where the windows prompt come up whenever i try to install it. So im basically down to windows defender, and it automatically is off and won't come on. Its saying the app has been turned off and isn't monitoring my computer. I need help installing the avast premier and turning windows defender back on for good. Thanks.