Hooking into Internet Explorer

[Argletoast]

I have been set with a task to write a simple internet content filter for internet explorer, something much like Net Nanny and other such programs.

I use the COM interfaces which Internet Explorer makes available, namely IWebBrowser2, and can successfully block and allow websites according to their URL.

My problem is that I would also like to give the option of disabling images from being displayed/downloaded.

After some research into the topic, I see that I must use an ICustomDoc interface, which is obtained by calling QueryInterface on a IHTMLDocument2 interface, requesting an IID_ICustomDoc.

When I do this QueryInterface always returns E_NOINTERFACE.

HRESULT IDispatch::Invoke(DISPID dispIdMember, REFIID riid, LCID lcid, WORD wFlags, DISPPARAMS *pDispParams, VARIANT *pVarResult, EXCEPINFO *pExcepInfo, UINT *puArgErr)
{
    LPDISPATCH              lpDispatch;

    switch(dispIdMember)
    {
        case DISPID_NAVIGATECOMPLETE2:
                lpDispatch   = pDispParams->rgvarg[1].pvarVal->pdispVal;
                hResult = lpDispatch->QueryInterface(IID_IWebBrowser2, (void**)&Web);
                hResult = Web->get_Document(&pHTMLDocument);
                hResult = pHTMLDocument->QueryInterface(IID_ICustomDoc, (LPVOID*)&CustomDoc);
                // hResult is always E_NOINTERFACE after this call.
        break;
    }
}

Can anyone suggest a solution here? See whats going wrong?