I read Mark's latest blog "The Case of My Mom's Broken Microsoft Security Essentials Installation", and it was an interesting (I can't say it was pleasant though) read. So the long story short is that Mark's mom bought a new PC, and she used a 3rd party software to migrate her data from her old computer to the new computer. That computer also mistakenly copied some registry settings that belong to Windows Installer's database containing entries on an an old instance of Microsoft Security Essentials she had installed on her old computer. These partly copied settings ended up corrupting the Windows Installer database. So Windows Installer thought it has already another instance of the software installed, but can't find it, nor it can find its components and other related information, leading to a wierd behavior.
This can happen to any Windows Installer based setup pogram, and not only Microsoft Security Essentials setup, but what Microsoft Security Essentials probably could do better, is provide a better is to lead the more savvy users to where it saves its diagnostics logs and information. So if Mark would have consulted the installation logs, it'd probably have been a shorter and more straight forward diagnostics journey.
The Microsoft Security Essentials setup saves its installation logs at: "%ProgramData%\Microsoft\Microsoft Security Client".
You can also download and run the Microsoft Security Support tool to collect the installation logs and relevant diagnostics information. The tool is available at: http://www.microsoft.com/download/en/details.aspx?id=13088
"Microsoft Security Support tool
The Microsoft Security Support tool gathers support data necessary to help the Microsoft Support team to resolve support issues.
Run The Microsoft Security Support tool on the problematic FEP computer (client or server).
Send the cab file that is created to the Microsoft Support team to enable them to diagnose and resolve the support issues."