Thank you very much! Given the above article and a description of the Windows PE format I managed to write my own functions for changing the import address table. Almost everything works. My function which overwrites the API address returns a pointer to the old function and it is the right GetOpenFileNameW pointer (I checked it with Dependency Walker). Now the problem: Notepad.exe does not seem to get my new function pointer right. It seems to point to garbage, at least notepad now crashes when clicking open. It was given in this way: &MyFunction. Why does it not work? Do I have to export my override function?