try
   if not Assigned(form) then
      form := TForm1.Create(Self);
   form.ShowModal;
finally
   form.Free;
   form := nil;
end;

procedure TForm1.Button1Click(Sender: TObject);
var
  h: HWND;
begin
  h := FindWindow(nil, 'Form1');
  if h <> 0 then
  begin
    if MessageDlg('Form1 Found!'#13#10'bring it to front?', mtConfirmation, [mbYes, mbNo], 0)=mrYes then
      SetForeGroundWindow(h);
  end
  else
    ShowMessage('Form1 not found!');
end;