Please see my comment to the question. Compile all the assemblies used in your process to the same target
instruction-set architecture (
http://en.wikipedia.org/wiki/Instruction_set[
^]).
Prefer using the target "AnyCPU", them the actual target will be determined by the platform when the application is loaded. If one of the components (.NET assemblies or unmanaged code modules) still have to be compiled for some particular architecture, the final architecture is defined by targeting the entry assembly of the application.
With Visual Studio, please see the project properties window.
—SA