It has been a few months since I installed Windows Live OneCare and results are mixed. For the most part it runs quietly in the background providing, at least I think, real-time protection. The problem the last few starts has been MsMpEng.exe (Windows Defender) crippling my machine with heavy CPU utilization. I've Googled, Live- Searched, MS-Knowledge Base searched and found basically the same thing... High CPU... but no resolution.
I dug through OneCare help and still didn't find anything. Therefore I decided to look in the registry and found: HKLM | One Care Protection | Exclusions. Under Exclusions I found three folders: Extensions, Paths, and Processes. I know through the One Care GUI I can designate folders and files for exclusion. I have excluded a few folders and found these in Paths. On a whim, I decided to add MsMpEng.exe in Processes. I was initially blocked lacking permission. I added full control and then added the DWORD: MsMpEng.exe having a data value of 0. I restored the permissions and restarted. After the restart, MsMpEng.exe has not consumed more than 50% CPU at startup (for only a brief time) and then occasionally 7-10% when browsing or starting other applications. For the most part it runs quietly at 0% I'm only guessing, but MsMpEng.exe may have been monitoring MsMpEng.exe. So far so good...