Hello,
I'm not familiar with DLLs, I would like to know how can I call a function in User32.dll ? (specifically, RegisterWindowMessage)
Thank you for any help!

You'll need a couple of things:

Link with the user32.dll import library, which is located in
DMD\dmd2\windows\lib\user32.lib.
Either pass it to DMD during compilation or more simply just can add a
pragma(lib) in your source file like so:
pragma(lib, "user32.lib");

And you need the function prototype. for windows api it needs the extern(Windows) calling convention. So your code would look like: http://codepad.org/hvOPpP9c

You are better off using the WindowsAPI bindings project, which includes almost all windows api function prototypes. You can download and read about it here: http://dsource.org/projects/bindings/wiki/WindowsApi

Thanks a lot!