Why do I need to overload =

Hi,

I am trying to transform a class with some time consuming operation by
adding a thread. To be able to pass data to thread I have declared a
class ThreadParam as shown below :

class CProvisioning
{
public:
    struct ThreadParam
    {
        ThreadParam(CProvisioning* a_pThis, const CString& a_strConfig,
CString& a_strXmlOut):
            pThis(a_pThis),
            strConfig( a_strConfig ),
            strOut( a_strXmlOut ) {}

        CProvisioning* pThis;
        const CString& strConfig;
        CString& strOut;
    };

    CProvisioning(void);
    ~CProvisioning(void);

    static DWORD ProvisioningThr(LPVOID pvarg);
    DWORD ProvisioningThr(const CString& strConfig, CString& strXmlOut);

    BOOL ProcessConfigXML(const CString& strConfig, CString& strOut);

protected:

    ThreadParam m_thrParam;
    HANDLE m_hProvThread;
};

CProvisioning::CProvisioning(void):
m_thrParam(this, CString(_T("")), CString(_T(""))),
m_hProvThread(NULL)
{
}

The ProcessConfig method receives two strings, one holding the config
file and the other to put the result:

BOOL CProvisioning::ProcessConfigXML(const CString& strConfig, CString&
strXmlOut)
{
    DWORD dwRet = 0;

         // Start a thread
    m_thrParam = ThreadParam(this, strConfig, strXmlOut);
    m_hProvThread = CreateThread(
        NULL,
        0,
        &CProvisioning::ProvisioningThr,
        &m_thrParam,
        0, NULL);
   ...
}

/*static*/
DWORD CProvisioning::ProvisioningThr(LPVOID pvarg)
{
    ThreadParam* pThrParam = static_cast<ThreadParam*>(pvarg);
    if (pThrParam) {
        CProvisioning* pThis = pThrParam->pThis;
        return pThis->ProvisioningThr(pThis->m_thrParam.strConfig,
pThis->m_thrParam.strOut);
    }

    return -1;
}

DWORD CProvisioning::ProvisioningThr(const CString& strConfig, CString&
strXmlOut)
{
    // Some lengthy operations ...

    return 0;
}

The problem I get is with m_thrParam = ThreadParam(this, strConfig,
strXmlOut); because I get :

error C2582: 'operator =' function is unavailable in
'CProvisioning::ThreadParam'

1) First I don't understand why I need to overload =
2) How can I fix it ?