Not enough information. But you should understand certain things. You should never ever mix different target platforms in one process. All platforms can be freely combined only with "AnyCPU". AnyCPU is the most usable target which you should use in almost all cases. It is possible because it is based on JIT compilation which happens during runtime. Any concrete platforms can be used only because of some dependency on some unmanaged executable module. If such modules are not used, all targets should be "AnyCPU".
Also, you should know that there are at least two different 64-bit targets incompatible with each other: x86-64 (also called AMD64) and IE-64 (also called "Itanium"). If, by some reason, you are trying to use an assembly which comes without source code and is targeted to "Itanium", you won't be able to use it on your system, if it has physical x98-64 CPU, no matter what you do, even if you install a x86 (32-bit) OS on it. But all physical CPU architectures are compatible with x86. Please see:
http://en.wikipedia.org/wiki/Instruction_set[
^],
http://en.wikipedia.org/wiki/x86[
^],
http://en.wikipedia.org/wiki/X86-64[
^],
http://en.wikipedia.org/wiki/Itanium[
^].
One more thing you should know is this:
http://en.wikipedia.org/wiki/WOW64[
^].
You can use any x86-targeted assemblies on your 64-bit system (reportedly, it works purely on Itanium, but I personally did not try), but you still cannot mix-up any different concrete architectures.
—SA