Detecting and preventing Windows shut down

When Windows is about to shut down, it sends a WM_QueryEndSession to all open applications. To detect (and prevent shutdown) , we must define a message handler to this message. Put this definition on the private section of the main form:

procedure WMQueryEndSession(var Msg : TWMQueryEndSession) ; message WM_QueryEndSession;

Also, to prevent Windows shutting down put this method in the implementation section of the unit:

procedure TForm1.WMQueryEndSession(var Msg : TWMQueryEndSession) ; 
begin 
	if MessageDlg('Close Windows ?', mtConfirmation, [mbYes,mbNo], 0) = mrNo then 
		Msg.Result := 0 
	else 
		Msg.Result := 1 ; 
end;

To detect Windows Shutdown, we must trap the WM_ENDSESSION message.

Declare a message handling procedure in your main Form's Private section:
 

Procedure WMEndSession(var Msg : TWMEndSession) ; message WM_ENDSESSION;

Also, add the following procedure to the implementation section of your Unit:

procedure TForm1.WMEndSession(var Msg : TWMEndSession) ; 
begin 
	if Msg.EndSession = TRUE then 
		ShowMessage('Windows is shutting down ' + #10#13 + 'at ' + FormatDateTime('c', Now)) ; 
 	inherited; 
end;