Starting with Windows ME/2000, applications not running in the foreground can not bring their windows to the front. You usually see their taskbar button blink when they are trying to do so, e.g. by invoking SetForegroundWindow().
Unfortunately one of our older applications was actually split in two processes, which communicate to each other using TCP sockets. Both applications have graphical user interface. As customers (who never really saw two applications but just one GUI) started to switch from Windows NT to 2000/XP, they noticed that newly created windows were overlapped or completely hidden by others. As a temporary workaround (which was only available under XP) they started the application in NT compatibility mode.
The remedy came with a new API-function, AllowSetForegroundWindow(). The foreground process can allow any other process to bring windows to the front. All it needs to know is the target process's id. And of course timing is crucial, once the invoking process is not the foreground process any more, it cannot allow other process to open foreground windows either. Clearly one has to check the proper windows version before invoking AllowSetForegroundWindow(). This can be done using GetVersion().