In addition to the case where Windows Defender was disabled by a group policy, some users said that they did not open Windows Defender with an unexpected error, or that Windows Defender cannot be opened after uninstalling Avast, Bitdefender, McAfee, AVG. However, many users have reported that their Windows Defender on Windows 10/8/7 cannot be enabled when they try to run this antivirus program by clicking the Power On button. Once it scans and finds potential threats, this program will stop them. And it provides real-time protection against various threats such as spyware, malware and viruses on your PC. Windows Defender (also called Windows Defender Security Center under Windor later), a fully integrated component of Windows, is an antivirus program. Update: Stop getting error messages and slow down your system with our optimization tool.