Windows Defender - Virus and spyware definitions couldn't be updated

When did Defender update?  What definitions are installed? Remember Defender should autoupdate appx every 24 hrs as needed.

https://www.microsoft.com/security/portal/definitions/whatsnew.aspx

See Troubleshoot Windows Defender Update Issues in Windows 10

Any other security program installed?

The behavior you are describing may occasionally occur if another antivirus program is installed or there are remnants of a third party antivirus program still installed

Go to Security and Maintenance, look in 'Security' and see what program is listed under Virus Protection.  Windows Defender will not be enabled and functioning properly if there is a program other than Windows Defender listed.  Suggest you uninstall any other AV programs on the computer and run the software removal tools for any security software (AVG, Baidu, Kaspersky, McAfee, Norton, whatever) that may have previously been on the computer (including a trial version of McAfee, Norton, Trend, Webroot, 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 install a third party antivirus 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/

Started having same problem on my PC today as well.  Not good...not good at all...

Windows Defender - Virus and spyware definitions couldn't be updated

I have a problem with installing updates and so what can I do for that. There is a maleware called win32/Powemet.B!attk and window defender can't kill it on my laptop.

See...

    • HOW TO begin your own new thread to avoid confusion

       https://answers.microsoft.com/en-us/page/faq#faqAskQuestion1

    • Suggestions for asking a question in help forums

       https://support.microsoft.com/en-us/kb/555375

why can't i update virus definitions?