Problem in Paint

Following is the code for the ON_WM_MOUSEMOVE in my paint program.

Code:

---

void CPainterView::OnMouseMove(UINT nFlags, CPoint point)
{ 
        CClientDC *pDC = new CClientDC (this);

        switch (flag)
        {
                case 1:
                        //drawing freehand
                        break;
                case 2:
                        //drawing line
                        if (nFlags && MK_LBUTTON)
                        {
                        pDC->SelectStockObject(WHITE_PEN);
                        pDC->MoveTo (Anchor.x,Anchor.y);
                        pDC->LineTo (OldPoint.x, OldPoint.y);
           
                        pDC->SelectStockObject (BLACK_PEN);
                        pDC->MoveTo (Anchor.x, Anchor.y);
                        pDC->LineTo (point.x, point.y);
                        }
                        break;
                case 3:
                        //drawing a rectangle
                        if (nFlags && MK_LBUTTON )
                        {
                                pDC->SelectStockObject (WHITE_PEN);
                                pDC->SelectStockObject(NULL_BRUSH);
                                pDC->Rectangle(Anchor.x,Anchor.y,OldPoint.x,OldPoint.y);

                                pDC->SelectStockObject(BLACK_PEN);
                                pDC->SelectStockObject(NULL_BRUSH);
                                pDC->Rectangle(Anchor.x,Anchor.y,point.x,point.y);
                                break;
                        }
               
        }
        OldPoint.x = point.x;
        OldPoint.y = point.y;
}

---

OldPoint is already initialised to Anchor which is the Starting point, where the user clicked originally.
The Problem is when I try to draw a new rectangle on an older one, some part of the older one gets erased. Why is this happening?
And what do I do to Stop it.
Thankx.