I have a Windows 7 i3 laptop that has worked well for many years. I have always liked Microsoft Security Essentials because it just worked. However this is no longer the case. I am embracing the cloud and have installed
- OneDrive for Business
- Microsoft Office 365 including OneNote etc
The sync activity associated with these products is causing MsMPEng.exe to take very high CPU and the laptop to become unusable. Adding the sync tool exes to the list of excluded locations helped a bit but not much.
I have un-installed Microsoft Security Essentials and custom installed Avast! Free with a minimum footprint (only shields and browser protection). This is working well at present.
— update 2015-07-11 —
I found that one reason for the high CPU usage was that my laptop fan was failing or blocked with dust. This was causing the CPU temperature to increase and the firmware was then then throttling it. This caused the CPU utilisation to increase and so on. I found this is the event logs but you should also try cleaning your GPU and CPU fans of dust anyway.