When receiving an error of this nature:

And the files are present in your file system "C:\Program Files\Windows Defender" (path to service)...

 TRY THIS :

1. Repairing Protected System Files:

Try running this at cmd prompt first :sfc  to see the options then sfc /scannow to check for and to try repair problems on all protected system files

if that does not work...

2. Ensure Path to Service is Correct:

First check the correct Service Name in the services app:(Select "Start button" then type"services"... scroll down to the name of the service and open it )

Open Regedit.

navigate to the service branch that is specied in the error - check what the name of the service is on the services... here it is "WinDefend"

Navigate to:

"HKLM\SYSTEM\CurrentControlSet\Services\WinDefend\Parameters"

ensure that the key "ServiceDll" has the correct path... if the services is 64 bit it should be "%ProgramFiles%" and not "%ProgramFiles(x86)%"

 Then restart your service.