Found this on MS forums:
> When trying to install Windows Defender, I was getting the following error
> "Service 'Windows Defender' (WinDefend) failed to start. Verify you have
> sufficient privileges to start system services."
>
> My problem was due to Spyware or a virus. I got rid of the Spyware by using
> AVG and MalWareBytes but was still not able to install Defender (also I had
> never had Defender installed on the PC).
>
> Here is how I fixed it:
> Searched the Registry for any "Defender" entries. Found an entry under
> "HKLM\Software\Microsoft\W
indows Defender". When I tried to delete it
> (Windows Defender folder) I found I could not due to not enough rights. So I
> right-clicked the folder selected "Permissions" for this folder and gave the
> Administrator FULL rights (it had only READ). I was then able to delete this
> registry folder.After this I was able to install Windows Defender without any
> problems. I believe that the Spyware or virus I got added this entry to the
> registry to prevent me from installing Windows Defender.
Also check out this:
http://smartnetadmin.blogspot.com/2010/08/error-1920-windows-defender-service.html