case WM_PAINT:
pDC = BeginPaint(hWnd, &ps);
// TODO: 在此添加任意绘图代码...
MoveToEx(pDC,m_Begin.x,m_Begin.y,NULL);
LineTo(pDC,m_End.x,m_End.y);

EndPaint(hWnd, &ps);
break;是不是要用到DPtoLP和LPtoDP?