Microsoft Security Essentials (MSE) are not essential. They do serve a purpose, but you may find yourself wanting them to not run. Here's how:

• Open the application 
  • click the MSE icon in the task-bar
  • find MSE from the start menu
  • navigate to or click start->run and enter "C:\Program Files\Microsoft Security Essentials\msseces.exe"
• From the control panel
• go to the ‘Settings’ tab 
• Un-tick the ‘Turn on real-time protection’ checkbox
• Microsoft Security Essentials is now disabled