Error LNK1561: entry point must be defined

It seems, that you misunderstand the term "module". There is no such C++ project in Visual Studio; C++ projects may be divided into three categories:

• Programs - compilation produces an exe file, which may be executed;
• Static libraries - compilation produces a lib file, which may be included in another project and are linked during the compilation;
• Dynamic libraries - compilation produces a dll file, which may be attached to your program at run-time and provide additional functionality.

From your description, you want the projectB and projectC to be a static libraries, but instead you created them as executable files. Run the new project wizard again and choose "static library" instead of "Windows application".

You can read more about static libraries in the MSDN library.

If static libraries are too heavyweight for your application, you may simply include projectB and projectC files in your project (optionally take care of namespaces not to confuse the names of classes). It all depends on how much functionality you plan to implement in these "modules".

set Properties -> Linker -> System -> SubSystem to "Windows (/SUBSYSTEM:WINDOWS)"

What's happening possibly, what was happening with me, is that when you switch your properties of your project to .dll from .exe, if you switch from debug to release or from x86 to x64, each time you do that it's switching you back to .exe. Each configuration has it's own properties.

So, go to Properties > Configuration Type > .dll

If indeed you want to make a .dll.