Defender will not open in Windows 8

[teejay330]

I have seen several threads with this same problem. However, I am still stuck on being unable to turn on defender even after following the instructions in previous threads. When I go to the action center and click turn on now, it goes to a list of files with which I have no idea what I am supposed to do. When I go to the control panel, I get the message that "this app has been turned off and is not monitoring your computer". Also, I know that defender can not be turned on when there is another security system running. I previously had a Norton free trial, but I believe I have removed all of the software. In addition, the action center shows that there is nothing protecting the computer. I am sure I am just doing something foolish, but I am completely stuck. Please help!