https://www.tenforums.com/tutorials/5918-turn-off-windows-defender-windows-10-a.html Option two should turn it on. As soon as you turn it on suggest you manually update Windows Defender – after updating run a manual Quick Scan. It should then be working okay unless another AV program or remnants of another AV program are causing issues.
Go to Security and Maintenance, look in 'Security' and see what program is listed under Virus Protection. Windows Defender (WD) will not be enabled and functioning properly if there is a program other than Windows Defender listed.
See the reply of quietman7 in https://answers.microsoft.com/en-us/protect/forum/protect_defender-protect_updating/windowsdefender-wont-turn-on/c502c63f-2ffe-4cfd-8b6b-bdfd086cfae0
And/Or
If you cannot resolve this issue with WD, you may want to try the steps recommended in Problems starting Windows Defender in Windows 8/8.1/10 to include (but not necessarily limited to) running SFC, scanning for malware as well as running the software removal tools for any security software (AVG, Baidu, Kaspersky, McAfee, Norton, whatever) that may have previously been on the computer (to include a trial version of McAfee, Norton, etc that may have been on the computer when it was new) followed by a restart of the computer. See List of anti-malware product removal tools
Alternatively - - -
You may prefer to use a third party antimalware program (which should disable Windows Defender).
There are a number of viable free and paid options available – take your pick…
Choosing an Anti-Virus Program
Is Windows Defender The Best For Windows 10? Good article but ignore the ads.
http://windows.microsoft.com/en-US/windows/antivirus-partners#AVtabs=win10
Regards…
http://blog.emsisoft.com/2015/01/27/top-10-ways-pups-sneak-onto-your-computer-and-how-to-avoid-them/