Critical section-Updating the Dialog from Workerthread

Hi every one,
I have a dialog application with a start button, a slider control, an
edit box where the value of the slider control will be displayed and
another edit box where the result of some operations made from a
workerthread will be displayed. When moving the slider, the new value
of the slider should appear in the corresponding edit box and the
workerthread should be aware that the value of the slider has changed
and should update the value of the slider in the calculations being
performed.

I created a class CWorkerThreadMgr (for the thread code) which
contains the followings :
    HRESULT Start (HWND hWnd) ; // from where the workerthread will
be started
    HWND m_hWnd ; //Window handle to the UI dialog used to post
messages in order to update
                                                    //the result of
the calculation in the edit box
    HANDLE m_hTread; //handle of the worker thread
    static UINT WINAPI ThreadProc( LPVOID lptest );

HRESULT Start (HWND hWnd)
{
    HRESULT hr=S_OK;
    m_hWnd=hWnd;
    m_hThread = (HANDLE)_beginthreadex(NULL,0,ThreadProc,
static_cast<LPVOID>(this), 0,NULL);

 return hr;
}

// Implementation in the stdafx.h
#define WM_SENDCALCULATED_VALUE WM_APP + 1

// Implementation in the CStartCalculationDlg.h

CWorkerThreadMgr m_WorkerThreadMgr //instance of the
WorkerThreadMgr
CSliderCtrl m_Slider //member variable of the slider control
CString m_SliderValue // member variable of the edit box, where the
current value of the slider control will be
                                   //displayed

CString m_Calculation // member variable of the edit box where the
calculated result from the workerthread will
                                  //be displayed via PostMessage
afx_msg LRESULT OnSendCalculatedValue( WPARAM, LPARAM );
afx_msg void OnHScroll(UINT nSBCode, UINT nPos, CScrollBar*
pScrollBar);

// Implementation in CStartCalculationDlg.cpp

BEGIN_MESSAGE_MAP(CStartCalculationDlg, CDialog)
=85
ON_WM_HSCROLL()
ON_MESSAGE(WM_SENDCALCULATED_VALUE, OnSendcalculatedValue)

void CStartCalculationDlg::OnHScroll(UINT nSBCode, UINT nPos,
CScrollBar* pScrollBar)
{
  m_SliderValue.Format(_T("%d"),m_Slider.GetPos());
  SetDlgItemText(IDC_SLIDER_VALUE,m_SliderValue);
}

BOOL CStartCalculationDlg::OnInitDialog()
{
      m_Slider.SetRange(0,50);
      m_Slider.SetTicFreq(5);
      SetDlgItemText(IDC_SLIDER_VALUE,_T("0"));
}

void CStartCalculationDlg::OnBnClickedStart()
{
   m_WorkerThreadMgr.Start(GetSafeHwnd());
}

static UINT WINAPI ThreadProc( LPVOID lptest )
{
 CWorkerThreadMgr* pCalculateMgr = reinterpret_cast< CWorkerThreadMgr*


//From the thread procedure a random number multiply by the slider
value
//should occur 40 times: rand() *m_SliderValue and the result
displayed in the edit box on the gui

for( UINT uCount = 0; uCount < 40; uCount++ ){

//Check if the slider has been moved
//if so, update the value of the slider for the calculation
....???

//pCalculateMgr-> rand()* m_SliderValue;

PostMessage(pCalculateMgr-> m_hWnd, WM_SENDCALCULATED_VALUE,0,0);
}

}

LRESULT CStartCalculationDlg::OnSendCalculatedValue( WPARAM, LPARAM )
{
    // m_SliderValue and m_Slider as critical section ??

      m_Calculation.Format(_T("%d"),rand()*m_SliderValue); //???
      SetDlgItemText(IDC_CALCULATION, m_Calculation);

     return 1;
}

So my questions are the following:
Since I'm not allowed to manipulate the control elements from the
workerthread, how can I implement the =93OnSendCalculatedValue=94
function?
I have to check by each iteration, if the slider value has been
changed, how can I implement it in the Threadproc or should it be
somewhere else?
How can I implement m_Slider and m_SliderValue as critical section? Or
do I need to create new variables in the CWorkerThreadMgr class?

Thank you

Carol