You posted in the wrong forum, David. You will want to go here:
http://social.answers.microsoft.com/Forums/en-Uk/category/mse (that's the UK URL...)
That said, the most likely reason for a CPU drain by MSMPENG is indeed a conflict. And it is usually other security software.
Prior Security Software?
Any security software still installed?
A possible reason for this problem is that you have conflicting security software installed fully or partially on the PC. Remove all other security software from the PC.
You may need to use a cleanup tool to remove your prior security software:
http://social.answers.microsoft.com/Forums/en-US/msestart/thread/407bf6da-c05d-4546-8788-0aa4c25a1f91
Be sure to review this post:
Check list for installing Microsoft Security Essentials
If that fails to resolve the problem, open an email support case:
To open an email support case, start here:
https://support.microsoftsecurityessentials.com/
-steve
~ Microsoft MVP Windows Live ~ Windows Live OneCare| Live Mesh|MS Security Essentials Forums Moderator ~