This message is sent when the user selects a command from the System menu, when the user selects the minimize or maximize box, or when the screen saver is preparing to blank the screen.
WORD wParam
Specifies the type of system command. Windows defines a series of system-command types. The Multimedia extensions add the following system-command type:
SC_SCREENSAVE
Screen saver will be invoked. Return a nonzero value to prevent the screen saver from being invoked; otherwise, pass the message to DefWindowProc.
DWORD lParam
Not used with the SC_SCREENSAVE command.
Depends on system-command type.
Refer to the Windows SDK documentation for a description of the other WM_SYSCOMMAND command types.