Hi Simon,
Are you running any other antivirus software apart from Windows Defender?
If you are then it is most likely that which has disabled Windows Defender from running and hence the update cannot work.
You'll need to in that case disable the autoprotect and firewall features of this other antivirus application first, then restart computer so you can then go turn on Windows Defender and then attempt to update again.
Assuming you have temporarily disabled another antivirus application and restarted computer:
- In search / cortana box type Security & Mainenance and click on it when it appears
- Click the drop arrow on Security banner then click to view installed apps
- Select Windows Defender and turn on
- Now run updates again
- Once done you can if you want to re-enable your other antivirus protection (noting of course this will disable Windows Defender again). Restart computer if you choose to do this to ensure your System is running properly and is protected.
Alternatively, clicking Start > Settings > Updates & security > in left pane Windows Defender > then scroll down and click Open Windows Defender may also re-activate it so long as you have not disabled this via Group Policy Editor or used a registry tweak to disable it.
By the way with Windows Defender open you can update it directly within the Windows Defender app rather than via Windows Update Service.
Regards,
Regedit32