What AntiVirus program do you use?
What is the path for MsMpEng.exe?
If you use MSE (MS Security Essentials), this exe may be for that.
Unfortunately MS has used the same file-names for some MSE and Windows Defender files...
I'm on a system right now without MSE or Windows Defender, so i'll have to get on a different system that uses MSE to confirm the file locations.
This system does not have an MsMpEng.exe running in Task Manager!