Microsoft Security Essentials adds a long Scan with Microsoft Security Essentials entry to the right-click context menu of Windows Explorer. I find this entry irritating because not only is it long, but in the rare chance that I need to scan some file or directory, I would rather do it from the Security Essentials application itself.
This entry can be easily removed using ShellExView. Look for the Extension Name EPP and disable it. (The description of this entry will be Microsoft Security Client Shell Extension.) You will need to logout and login for this change to take effect.
For more information on removing other context menu entries, see this post.
Tried with: Microsoft Security Essentials 4.0.1526.0 and ShellExView 1.70