Not sure why you have code like this to open a word document i believe you are building a asp.net web application that the users can use remotly.
If my assumption is true even your code works on your local machine it wont work for the users when finally you will deploy it on production server as it will open the word document on server and return nothing to user.
in a client-server appilcation for a request web application must send some data as response with valid MIME type so that client browser can take appropriate action. having that said i believe you should update your code so that user can download the file i.e. open some file on clicking of soem button or link.
see below link:
http://stackoverflow.com/questions/2519026/how-do-you-stream-an-excel-2007-or-word-2007-file-using-asp-net-and-c-sharp[
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If you are trying to edit a word doc file in your asp.net file then that is a different story.