About a window with NULL_BRUSH

Dear all:
    In my code , I created a Window whose background is NULL_BRUSH , and
it's no menu and title . That is , it is a Transparnt window . And I created
a Timer to updatewindow.When the WM_PAINT message come , I DrawText on the
window.Every time I draw different text on the window.

    But now , I have a problem.
    The window isn't redraw.The text drawn at last time is'nt erased.So my
window will be too bad.
    What should I do?

    Some code is here:

VOID CTextWindow::WindowClass()
{
 m_wcWNDCLASS.style = 0; //Caution: CS_HREDRAW or CS_VREDRAW styles often
introduce flicker.
 m_wcWNDCLASS.lpfnWndProc = CTextWindow::StaticWindowProc;
 m_wcWNDCLASS.cbClsExtra = 0;
 m_wcWNDCLASS.cbWndExtra = 0;
 m_wcWNDCLASS.hInstance = 0;
 m_wcWNDCLASS.hIcon = NULL;
 m_wcWNDCLASS.hCursor = LoadCursor(NULL, IDC_ARROW);
 m_wcWNDCLASS.hbrBackground = (HBRUSH)GetStockObject(HOLLOW_BRUSH);
 m_wcWNDCLASS.lpszMenuName = NULL;
 m_wcWNDCLASS.lpszClassName = m_szClassName;
}

LRESULT CTextWindow::WndProc(HWND hWnd, UINT uMsg, WPARAM wParam, LPARAM
lParam)
{
 PAINTSTRUCT ps;
 HDC hDC ;
 switch (uMsg)
 {
 case WM_PAINT:
  hDC = ::BeginPaint(hWnd, &ps);
  OnPaint(hDC);
  ::EndPaint(hWnd, &ps);
  break;

 case WM_COMMAND:
 case WM_MOUSEMOVE:
 case WM_LBUTTONUP:
 case WM_LBUTTONDOWN:
 case WM_TIMER:
  OnCommand(uMsg ,wParam, lParam);
  break ;

 case WM_NOTIFY:
  OnNotify(wParam, lParam);
  break ;

 case WM_DESTROY:
  OnDestroy();
  break;

 default:
  if (m_pWindowProc)
  {
   return ::CallWindowProc(m_pWindowProc , hWnd , uMsg , wParam , lParam );
  }
  else
  {
   return ::DefWindowProc(hWnd, uMsg, wParam, lParam);
  }
 }
 // Now hand all messages to the default process
 if (m_pWindowProc)
 {
  return ::CallWindowProc(m_pWindowProc , hWnd , uMsg , wParam , lParam );
 }
 else
 {
  return ::DefWindowProc(hWnd, uMsg, wParam, lParam);
 }

}

BOOL CTextWindow::OnPaint(HDC hDC)
{
 RECT rect;
 TCHAR szText[MAX_PATH+1] = _T("\0");
 int i = 0 ;
 CircularListNode *p;
 ::GetClientRect(m_hWnd , &rect);
 SetBkMode(hDC , TRANSPARENT);

 p = m_DrawFirst;
 while (p != m_DrawLast)
 {
  szText[i] = p->text;
  p = p->next;
  i++;
 }
 szText[i+1] = m_DrawLast->text ;
 szText[i+2] = _T('\0');

 DrawText(hDC , szText , -1 , &rect ,DT_TOP | DT_LEFT | DT_SINGLELINE );

 return TRUE;
}

BOOL CTextWindow::OnCommand(UINT uMsg , WPARAM wParam, LPARAM lParam)
{
 if (uMsg == WM_TIMER)
 {
  switch(wParam)
  {
   case SPACETIMER:
    m_DrawFirst = m_DrawFirst->next;
    m_DrawLast = m_DrawLast->next;
    InvalidateRect(m_hWnd , NULL , TRUE);
    break;
   default:
    break;
  }
  return TRUE;
 }
 return TRUE;
}