活动窗口上的Windows系统事件是否已更改？

是的，您可以使用SetWinEventHook函数。

hEvent = SetWinEventHook(EVENT_SYSTEM_FOREGROUND ,     EVENT_SYSTEM_FOREGROUND , NULL,     WinEventProcCallback, 0, 0,     WINEVENT_OUTOFConTEXT | WINEVENT_SKIPOWNPROCESS);.......VOID CALLBACK WinEventProcCallback ( HWINEVENTHOOK hWinEventHook, DWORD dwEvent, HWND hwnd, LONG idObject, LONG idChild, DWORD dwEventThread, DWORD dwmsEventTime){    }