GetClientRect(hWnd, &rt);
int DlgWidth     = rt1.right - rt1.left;
int DlgHeight    = rt1.bottom - rt1.top ;

SetWindowPos(hDlg, 0, 0, 0, DlgWidth/2, DlgHeight /2, 
              SWP_NOZORDER | SWP_NOSIZE);