It's kind of too bad that doesn't exist anymore. If you want to have it so the user double clicks on a file in Windows Explorer and have the file open in an already existing instance of your application, this can be somewhat tricky. Windows file associations work by launching your program with the file name as a command line argument, so it will just keep launching instances of your application. So to get around this, when your application starts, it must somehow figure if there is already an existing instance, and if one exists, pass the command line arguments to the first instance to open the file, then the duplicate instance can exit. This is actually non-trivial, and one of the common solutions is to use DDE, which is basically unchanged since Windows 3.1.