If your computer is running slowly and you look in your task manager (Ctrl-Shift-Escape) and go to the Processes tab and find a process called MsMpEng.exe is using a lot of your CPU cycles then this is because you have Microsoft Security Essentials (MSE) installed to protect your computer.
Some websites describe how to disable Microsoft Security Essentials but this is a very bad idea unless you are already running another antivirus program.
A better solution is to change the time at which MSE performs its scheduled scans by opening the MSE control panel (double click its icon in the system tray) and going to the settings tab.