MsMpEng.exe is the back-end of Microsoft Security Essentials. It is important to remember that this process is actually used by both Windows Defender and MSE. When you install Microsoft Security Essentials, then Windows Defender is automatically turned off and Microsoft Security Essentials uses this process. This is the main process that runs the program and takes quite some system memory. If you end this process then Microsoft Security Essentials will be disabled and an alert will be shown aski

CPU
Total CPU:	0.14786202%	0.028634%
Kernel CPU:	0.07855999%	0.013761%
User CPU:	0.06930204%	0.014873%
Kernel CPU time:	19,400 ms/min	100,923,805ms/min
User CPU time:	169 ms/min	0 ms/min
Context switches:	86/sec	284/sec
Memory
Private memory:	53.26 MB	21.59 MB
Private (maximum):	37.83 MB
Private (minimum):	26.59 MB
Non-paged memory:	53.26 MB	21.59 MB
Virtual memory:	137.13 MB	140.96 MB
Virtual memory (peak):	194.35 MB	169.69 MB
Working set:	33 MB	18.61 MB
Working set (peak):	106.37 MB	37.95 MB
Page faults:	299,002/min	2,039/min
I/O
I/O read transfer:	6.84 MB/sec	1.02 MB/min
I/O read operations:	1,068/sec	343/min
I/O write transfer:	275.72 KB/sec	274.99 KB/min
I/O write operations:	34/sec	227/min
I/O other transfer:	37.33 KB/sec	448.09 KB/min
I/O other operations:	1,380/sec	1,671/min
Resource allocations
Threads:	17	12
Handles:	315	600
GUI GDI count:	4	103
GUI USER count:	2	49

ntdll.dll
Total CPU:	2.12504425%	0.272967%
Kernel CPU:	0.12400455%	0.107585%
User CPU:	2.00103970%	0.165382%
Context switches:	21/sec	79/sec
Memory:	712 KB	1.16 MB
offreg.dll
Total CPU:	0.00076478%
Kernel CPU:	0.00076478%
User CPU:	0.00000000%
Memory:	60 KB
MsMpEng.exe (main module)
Total CPU:	0.00074191%
Kernel CPU:	0.00072400%
User CPU:	0.00001791%
Memory:	16 KB
mprtplug.dll
Total CPU:	0.00021259%
Kernel CPU:	0.00015944%
User CPU:	0.00005315%
Memory:	60 KB
MpSvc.dll
Total CPU:	0.00009750%
Kernel CPU:	0.00004900%
User CPU:	0.00004850%
Memory:	272 KB