I know its not what you want to hear, but managed interop with MSXML has 
never been supported by the team, or ever recommended as a possible 
solution.

<mrjaxon@g...> wrote in message 
news:1178299721.619902.148920@e......
>I have a C# web application which leverages MSXML that I am trying to
> migrate to a 64 bit environment.  Currently the application is built
> on the .NET 2.0 Framework and using MSXML 6 (though I had the same
> issues using version 4).  The OS I am running the app on is Windows
> Server 2003 x64.  I am using the x64 version of MSXML 6 on the
> server.  My dev environment is Windows XP 32 bit SP2.  I have tried to
> compile the projects for "anycpu" and "x64" with the same results.
> The Interop.MSXML2.dll was generated by my dev environment
> automatically when I added the MSXML 6 reference to my project.
>
> Essentially whenever the program tries to instantiate a
> FreeThreadedDOMDocument40, it throws an exception with the message,
> "Object reference not set to instance of an object".  I have tried to
> use MSXML 4 and 6 (both are installed on the server) with the same
> results.  The line of code is:
>
> cXMLDOM = new FreeThreadedDOMDocument40Class();
>
> The program seems to be able to find the Interop.MSXML2.dll (I used
> FuselogVW.exe to confirm it), but it is still unable to find the
> necessary DLLs in the system directories.  The msxml6.dll and
> msxml6r.dll are present in both the system32 and SysWOW64 directories,
> and the application works fine in a 32bit environment.
>