MsMpEng.exe is the antispyware component of Microsoft Defender antivirus software suite. It's also part of the commercial Microsoft ForeFront antivirus software suite.
I run a Intel Core i5 M520 dual core processor running at 2.40 GHz, however, when MsMpEng.exe kicks in, the CPU utilization spikes to 100%. The system becomes completely unresponsive until MsMpEng.exe completes.