Please check up that all your modules are compiled to the target instruction-set architecture
"AnyCPU" and you don't have any native (non-managed) modules. If you do have some, you would need to provide a mechanism for loading alternative implementations for different instruction-set architectures, such as x86-64 or Itanium (IE-64). This is how native modules of OS work: on a different version of OS, you have the same but API compiled to the instruction-set architecture used on this system.
Alternatively, you can use all modules in x86, which is supported via WoW64: http://en.wikipedia.org/wiki/WOW64
In this case, you should make sure that you don't have a mixture of different instruction-set architectures in one process (different processes, naturally, can run in different platforms, WoW64 and the one native on the version of the OS. Again, make sure your native (non-managed) modules are consistent.
Generally, you need to have everything built to "AnyCPU", but, it working on WoW64 or any specific instruction-set architecture is important by some reason (which is always related to the unmanaged modules), and only the entry-point assembly should define a particular architecture.