If you are referring to the assemblies you are installing, you need to sign them before building the installer. Visual studio can sign, or you can set up a post build step to sign them manually with signcode.
I recommend visual studio.
http://msdn.microsoft.com/en-us/library/9sh96ycy(v=vs.80).aspx
http://msdn.microsoft.com/en-us/library/ms247123(v=vs.90).aspx