Windows Defender won't turn on after uninstalling antivirus I recently uninstalled ESET NOD32 antivirus and now Windows Defender won't turn on. Here are the following steps I took. 1. I uninstalled the antivirus through Control Panel > Uninstall a program. 2. Tried turning on Windows Defender in the Action Center but nothing happened when I clicked on "Turn on now". It still stayed off. 3.