CoMarshal.... working in NT, Not working in XP !!!

Problem:-

I have created a multi-threaded application which works without any problems on a NT-4.0 Workstation/Server. When I try to run the same application in Windows XP, I get an error in a call to CoMarshalInterThreadInterfaceInStream which returns -2147418113.
I have provided a snippet of the code below where the call fails in Windows XP.
Environment - Windows-XP,SP-2,Visual Studio 6.0,SP-4,ATL-3.0
Should I be doing anything different in Windows XP?

HRESULT hr = S_OK;
IUnknown** pp = p->m_vec.begin();
while (pp <>m_vec.end() && hr == S_OK)
{
if (*pp != NULL)
{
IEvent* pEvent = (IEvent*)*pp;
IStream* pIStream;
HRESULT hr = CoMarshalInterThreadInterfaceInStream(IID_IEvent, pEvent, &pIStream);
if(SUCCEEDED(hr))
{
CComPtr pMarshalEvent;
hr = CoGetInterfaceAndReleaseStream(pIStream, IID_IEvent, (void**)&pMarshalEvent);

if(SUCCEEDED(hr))
hr = pMarshalEvent->NewCurrentCassette(m_pCurrentCassette, m_setBy);
}
P++;
}

Thread 2:-

I remember facing this problem long time back.The reason it happened was b'cos of the Free-Threaded marshaller code in Finalconstruct and FinalRelease even though i don't remember the logic behind it.In my case commenting the Free-Threaded marshaller code did the trick.

1) The commented code in FinalConstruct was
hr = CoCreateFreeThreadedMarshaler( GetControllingUnknown(), &m_pUnkMarshaler.p);
PROCESS_HR(IID_ISomeThing);

2)In FinalRelease it was the corresponding m_pUnkMarshaler.Release(); that was commented.

3)In the header,DECLARE_GET_CONTROLLING_UNKNOWN() and COM_INTERFACE_ENTRY_AGGREGATE(IID_IMarshal, m_pUnkMarshaler.p) and CComPtr m_pUnkMarshaler; was commented.

4)Remove marshalling code i.e,CoInterface and related marshalling code.The interface pointer can be accessed in the secondary thread directly,no need of marshalling.

I remember faintly that Free-Threaded marshaller is basically to optimize marshalling.So in my case removing it did not have any side-effects as we were not worried about Free-Threaded marshaller.Again the above fix might work but the best thing to do will be to anaylze the apartment link(STA,MTA etc.) between say the client and the component and then come to a conclusion.

Thread 3:-

You need not marshal/unmarshal to call a method on the interface pointer since the sink class itself deriving from the IConnectionPointImpl takes care of unmarshalling. You can see the code in your connection point implementation class.

Thread 4:-

I don't think IConnectionPointImpl class as such has anything to do with marshalling, it is the m_pUnkMarshaler member object.It is the call to CoCreateFreeThreadedMarshaler in FinalConstruct that initializes the m_pUnkMarshaler object.I suggest reading the documentation about CoCreateFreeThreadedMarshaler in order to come to a conclusion whether to use it or not.By default ATL provides the code calling CoCreateFreeThreadedMarshaler API to do efficient marshalling across *thread of the same process(Refer doc)*, but depending on our need we may or may not use it.In my case we did not need it so we commented it out.It depends on the need,but generally i think it is safe to comment it out if we are going to access interface pointers in secondary threads.Hope this helps.