I spent ages trying to figure out this problem, when trying to reference Microsoft Office Interop libraries in my project (For manipulating an Excel file programmatically).
Turns out, the dlls I was trying to reference were 32-bit, and the platform my project was set to run on was 64-bit. As I was building this project on a 64-bit machine, Visual Studio set the default platform to be 64-bit.
This is a simple fix: Go to the “Project” menu, select “Properties”, then click the “Build” tab on the side. From here you just need to select x86 from the Platform combo box and rebuild your project. Sorted!