Оформил: DeeCo

function CenterProc(nCode: Integer;
  wParam: WPARAM;
  lParam: LPARAM): LRESULT; stdcall;
begin
  Result := CallNextHookEx(hHook, nCode, wParam, lParam);
  if nCode = HCBT_ACTIVATE then
  begin
    //    CenterDialog(wParam);
    UnhookWindowsHookEx(hHook)
  end
end;

function MessageBoxCenter(hWnd: HWND; lpText, lpCaption: PChar; uType: UINT):
  Integer;
var
  hHook: THandle = 0;
begin
  hHook := SetWindowsHookEx(WH_CBT, @CenterProc, hInstance, GetCurrentThreadID);
  Result := MessageBox(hWnd, lpText, lpCaption, uType);
end;