Hi Rich,
If MSMPENG.EXE is constantly consuming excessive memory and CPU, it should be caused conflicts between services and process. I recommend you tring the Dichotomy method to find out the root cause. you can refer to the KB929135, I have included the link for your convenience:
http://support.microsoft.com/kb/929135/
In addition, you can use the Process Monitor v2.8 to monitor what's the MSMPENG.EXE process doing.
If you find this issue is caused by the Microsoft Security Essentials, you can shutdown all task in the Microsoft Security Essentials, or reinstall it.
I hope this can help you.
John