[Microsoft Microsoft Security Essentials 1]

If installing via Microsoft Deployment Toolkit, see the following useful article about ensure that the correct architecture (32-bit vs 64-bit) version is installed automatically.

http://deployment.xtremeconsulting.com/2009/10/30/how-to-filter-out-platform-specific-applications/

In my deployment, I have included the 32-bit app, 32-bit defs, 64-bit app and 64-bit defs all as separate hidden applications, all in one bundle.