The WM_USERCHANGED message is sent to all windows after the user has logged on or off. When the user logs on or off, the system updates the user-specific settings. The system sends this message immediately after updating the settings.
WM_USERCHANGED
wParam = 0; // not used, must be zero
lParam = 0; // not used, must be zero
Return Values
An application should return zero if it processes this message.