You install Windows Live OneCare. When you manually start Windows Defender, you may receive an error message that resembles the following message:
Windows Defender is turned off. Windows Defender will not provide protection against harmful or potentially unwanted software and it will not send you alerts because it is off. To help protect your computer against harmful or potentially unwanted software, turn on and open Windows Defender.
This issue occurs because Windows Defender is disabled when you install Windows Live OneCare. Windows Live OneCare determines whether Windows Defender is enabled whenever you start the computer. If Windows Live OneCare finds that Windows Defender is manually enabled, Live OneCare will disable Windows Defender.
Note Windows Live OneCare includes the antispyware component of Windows Defender. Therefore, Windows Defender does not have to be running when Windows Live OneCare is running.