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

const
   mbMessage = WM_USER + 1024;

 type
   private
      procedure ChangeMessageBoxPosition(var Msg: TMessage); message mbMessage;
   end;


 var
   Form1: TForm1;
   msgCaption: PChar;  // variable to hold the caption 

implementation


 {$R *.DFM}

 procedure TForm1.ChangeMessageBoxPosition(var Msg: TMessage);
 var
   MbHwnd: longword;
   MbRect: TRect;
   x, y, w, h: integer;
 begin
   MbHwnd := FindWindow(MAKEINTRESOURCE(WC_DIALOG), msgCaption);
   if (MbHwnd <> 0) then
   begin
     GetWindowRect(MBHWnd, MBRect);
     with MbRect do
     begin
       w := Right - Left;
       h := Bottom - Top;
     end;
     // center horzontal 
    x := Form1.Left + ((Form1.Width - w) div 2);
     // keep on screen 
    if x < 0 then
       x := 0
     else if x + w > Screen.Width then x := Screen.Width - w;
     //center vertical 
    y := Form1.Top + ((Form1.Height - h) div 2);
     // keep on screen 
    if y < 0 then y := 0
     else if y + h > Screen.Height then y := Screen.Height - h;
     // set new windows position 
    SetWindowPos(MBHWnd, 0, x, y, 0, 0, SWP_NOACTIVATE or SWP_NOSIZE or SWP_NOZORDER);
   end;
 end;


 procedure TForm1.Button1Click(Sender: TObject);
 begin
   PostMessage(Handle, WM_USER + 1024, 0, 0);
   msgCaption := 'Confirm';
   MessageBox(Handle, 'Has our MessageBox moved ?', msgCaption,
     MB_ICONQUESTION or MB_YESNO);
 end;