painting in SDI application

hi experts, I have a problem that is really vaxing me much.I need to
draw a rectangle in a way we draw it in microsoft paint that is by
dragging mouse.My problems are :

1) i get black background and white rectangle. I dont want black
background. but i dont find any way in solving this problem .

2) My previous rectangle gets erased when i try to draw new rectangle.
My code is

class CDerivedWnd : public CWnd
{
       public:
    bool IsLButtonDown;
    bool IsMouseMove;
    CPoint pointFirst,pointPrev,pointCurr;
    CDC* dcDerivedWnd;
}

void CDerivedWnd::OnLButtonDown(UINT nFlags, CPoint point)
{
    CWnd::OnLButtonDown(nFlags, point);
    IsLButtonDown=true;
    pointFirst.x=point.x;
    pointFirst.y=point.y;
    pointPrev.x=point.x;
    pointPrev.y=point.y;
    pointCurr.x=point.x;
    pointCurr.y=point.y;

}

void CDerivedWnd::OnLButtonUp(UINT nFlags, CPoint point)
{
    CWnd::OnLButtonUp(nFlags, point);
    if(IsMouseMove)
    {
                 pointCurr.x=point.x;
                    pointCurr.y=point.y;
                 IsMouseMove=false;
    }

    IsLButtonDown=false;
}

void CDerivedWnd::OnMouseMove(UINT nFlags, CPoint point)
{
    CWnd::OnMouseMove(nFlags, point);
    if(IsLButtonDown)
    {
        pointPrev.x=pointCurr.x;
        pointPrev.y=pointCurr.y;
        IsMouseMove=true;
        pointCurr.x=point.x;
        pointCurr.y=point.y;
    }
}

void CDerivedWnd::OnPaint()
{
// new rectangle to be drawn and old rectangle to be erased
    CRect RectNew(pointFirst,pointCurr);
    CRect RectOld(pointFirst,pointPrev);

// buffer DC
    CDC dcMem;

// rectangle structure to get current coordinates

    RECT RectStruct;

///Bitmaps of buffer dc and old bitmap
    CBitmap BmMem;
    CBitmap *pBmOld;

// Cpens
    CPen penWhite,penBlack;
    CPen *penOld;
//CBrushes
    CBrush brushWhite;
    CBrush *brushOld;

                 dcDerivedWnd=this->GetDC();

    dcMem.CreateCompatibleDC(dcDerivedWnd);

    this->GetClientRect(&RectStruct);

    BmMem.CreateCompatibleBitmap(dcDerivedWnd,RectStruct.right-RectStruct.left,RectStruct.bottom-RectStruct.top);

                 pBmOld=(CBitmap *)dcMem.SelectObject(&BmMem);

    penBlack.CreatePen(PS_SOLID,0,RGB(0,0,0));
    penWhite.CreatePen(PS_SOLID,0,RGB(255,255,255));

    brushWhite.CreateSolidBrush(RGB(255,255,255));

    if(IsMouseMove==true && IsLButtonDown==true)
    {
        penOld=(CPen*)dcMem.SelectObject(&penBlack);
        brushOld=(CBrush*) dcMem.SelectObject(&brushWhite);

        dcMem.Rectangle(&RectNew);

        dcMem.SelectObject(&penWhite);
        dcMem.FillRect(&RectOld,&brushWhite);

        dcMem.SelectObject(penOld);
        dcMem.SelectObject(brushOld);

dcDerivedWnd->BitBlt(0,0,RectStruct.right-RectStruct.left,RectStruct.bottom-RectStruct.top,&dcMem,0,0,SRCCOPY);

    }

                dcMem.SelectObject(pBmOld);

    this->ReleaseDC(dcDerivedWnd);
}

BOOL CDerivedWnd::OnEraseBkgnd(CDC* pDC)
{
    return false;
    //return CWnd::OnEraseBkgnd(pDC);
}

"PLZ TELL ME WHERE I M MISTAKEN
THANKS"