Just wanted to point out this updated Kb article (2435760) and associated Microsoft Fixit 5035

This article describes how to uninstall Microsoft Security Essentials if you cannot uninstall it in Control Panel by using the Add or Remove Programs item or the Programs and Features item in Windows Vista and Windows 7. We recommend that you verify that you cannot uninstall by using Add or Remove Programs first.”

Microsoft Fixit 50535

Read the full article at http://support.microsoft.com/kb/2435760/en-us

There is also a KB for how V2 of Microsoft Security Essentials if you are experiencing the same problem (KB 2483120)