I upgraded to Windows 10 from Windows 7 Ultimate on my laptop. Now, I cannot turn on Windows Defender. It works normally on my desktop with an earlier upgrade from W 8.1 to W 10. What can I do to start using Windows defender?

I assume you don't mean this question from a ConfigMgr perspective, but from a user perspective on Windows 10. In that case, you can turn the real-time protection on or off via Settings > Update & Security > Windows Defender.

Thank you for your kind reply. However, that is exactly my problem: nothing in Windows Defender there is highlighted. When I hit the last line "Use Windows Defender" I'm told that the app is turned of and is not monitoring my system. I do have Spybot running, but that does not prevent Windows Defender from working on my desktop, where it started automatically after the upgrade from W 8.1 to 10. My laptop upgrading did not turn the app on, and I still don't know how to get life in it ...

Is the Windows Defender Service service running?

I found this had some helpful information for when windows defender isn't working.

http://www.thewindowsclub.com/windows-defender-is-turned-off