You won't find proper option (in Microsoft Security Essentials configuration) for removing
Scan with Microsoft Security Essentials... menu item, that is added after each Microsoft Security Essentials installation. So, you have to help yourself.
The quickest way of getting rid of that option is to execute following sequence of commands:
cmd /c reg DELETE HKCR*shellexContextMenuHandlersEPP /f && reg DELETE HKCRDirectoryshellexContextMenuHandlersEPP /f && reg DELETE HKCRDriveshellexContextMenuHandlersEPP /f
(either after opening
Start menu and putting it to
Search program and files... field or by using
Start > Run... command)
The idea is from Twig's Tech Tips blog.
If manual remove is not an option for you, consider using ShellExView. This is a shell extensions manager, a small tool, that let you remove virtually anything from your right-click menu.