I had the same problem trying to build and run a .NET application on Windows 7 x64 that called interop.domino.dll, which is 32 bit only.
To resolve, I recompiled the .NET application to run specifically as x86 when run on x64 operating systems.
I was using Visual Studio 2010 Express Edition which is trickier to target specifically for x86 platforms than the paid for versions.
The solution was:
Click TOOLS > OPTIONS > PROJECTS AND SOLUTIONS
Check the box "Show advanced build configurations" and click OK
Click TOOLS > SETTINGS > check EXPERT SETTINGS to see the build configuration manager
Click BUILD > CONFIGURATION MANAGER select the platform dropdown to X86 and click CLOSE
Now rebuild the project
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