how do I reinstall windows defender for windows 10

not working,the window defender service, service on local computer started and then stopped.some services stop automatically if they are not in use by other services or program.

neither of these worked, despite having unistalled AVG antivirus.  The only other thing on my computer is the windows firewall which I think is still functioning.  so at this moment I have no protection except the firewall.  I have looked all over the Microsoft site & it is awful trying to find answers.

I came across a note (sorry...can't recall where) that said Defender would turn itself off if it encountered another anti-virus program (or maybe remnants of one?). It didn't say anything about turning it back on. That might be a clue. I still have no idea how to "reinstall" Defender.

Dear Lucy et al,

It would appear, reading through all the 32 replies thus far: It might be Windows Defender thinks that you are being looked after by another anti virus app. or in your case a bad choice of finger stab. Because Defender is glued and nailed into the win 10 OS and does not function as a separate app might, the only way to clean it up and make it work is with a reinstall of Win 10.

This can be done automatically from the action centre found on the right hand side of the ribbon at the bottom of the screen. Hovering over each icon look for one that says something about notifications.

From there L click All settings>update and security>Recovery>Reset this PC.

It gives you the option of saving your old files.

Before you start all this do get rid of any third party antivirus program and back up your files to an external media, also clean up your Edge downloaded items and history too just in case they get carried over.  

Let us all know how it goes, good luck.

No I don't get any other messages - defender just keeps turning itself off after I turn it on. In services.msc it status is manual. There was also a "defender network" app which had a status of manual - no idea what this is but that one I could at least turn on. I only just upgraded to Windows 10. Prior to the upgrade I wasn't using defender I was using micro titanium but that seems to have disappeared as well