There are several ways. One quick-n-dirty method is handle the WM_NCHITTEST message and return HTCAPTION for the places where you want the user to be able to drag (i.e., pretend your window has a caption when it doesn't).

For someone who wants to get nitty-gritty and cut the code themselves (and doesn't care about the inefficiencies involved), the code below demonstrates dragging using the right mouse button :

void CTestbed2Dlg::OnRButtonDown(UINT nFlags, CPoint point)
{
   ClientToScreen (&point);
   m_LastPoint = point;
   m_bMoving = TRUE;
}

// ------------------------------------------------------------------------

void CTestbed2Dlg::OnMouseMove(UINT nFlags, CPoint point)
{
   RECT cr;
   ClientToScreen (&point);

   if (m_bMoving)
   {
      if ((point.x != m_LastPoint.x) ||
          (point.y != m_LastPoint.y)   )
      {
         GetWindowRect (&cr);
         int iX = cr.left - (m_LastPoint.x - point.x);
         int iY = cr.top  - (m_LastPoint.y - point.y);
         int iWidth = cr.right - cr.left;
         int iHeight = cr.bottom - cr.top;

         MoveWindow (iX, iY, iWidth, iHeight);
      }
      m_LastPoint = point;
   }
   else
   {
      CDialog::OnMouseMove(nFlags, point);
   }
}

// ------------------------------------------------------------------------

void CTestbed2Dlg::OnRButtonUp(UINT nFlags, CPoint point)
{
  if (m_bMoving)
     m_bMoving = FALSE;
  else
     CDialog::OnMButtonUp(nFlags, point);
}

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