Оформил: DeeCo
Автор: http://www.swissdelphicenter.ch

{ 
  How to popup the windows system menu? 
  Maybe you can use Keybd_event to eumlate ALT+SPACE 
  Maybe you can use a TPopupmenu. 
  But they always have some problem. 
  The method below is a perfect solution! 
  BTW: if your form has borderstyle = bsNone, Please do it like this: 
  Set forms style = bsSingle; and use the code below to set form boder: 
  SetWindowLong(Handle, GWL_STYLE,GetWindowLong(Handle, GWL_STYLE) 
  and (not WS_CAPTION) or WS_DLGFRAME or WS_OVERLAPPED); 
}

 procedure TForm1.Button1Click(Sender: TObject);
 const
   { Undocument message ID }
   WM_POPUPSYSTEMMENU = $313;
 begin
   SendMessage(Handle, WM_POPUPSYSTEMMENU, 0,
     MakeLong(Mouse.CursorPos.X, Mouse.CursorPos.Y));
 end;