System.AccessViolationException when open docx

See more:

C#

I try to convert a docx to doc, using MS Office Word's "save as" function.
I write a simple testing code like below,
but it always meet a 'System.AccessViolationException:Attempted to read or write protected memory. This is often an indication that other memory is corrupt.' during runtime.
Does anyone know why the exception would occur??
Please give me some advice, thanks!!

p.s. I am not sure whether it occurs because my domain account doesn't have full privilege on disk C. I can't open the MSOCache folder in C.

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using Microsoft.Office.Core;
using Microsoft.Office.Interop.Word;
namespace _2007Convert2003
{
    class Program
    {
        static void Main(string[] args)
        {
            try
            {
                Microsoft.Office.Interop.Word.ApplicationClass oWord = new ApplicationClass();
                object oMissing = Type.Missing;
                object fileName = @"D:\Test\Combo.docx";
                Document oDoc = oWord.Application.Documents.Open(ref fileName, ref oMissing, ref oMissing, ref oMissing, ref oMissing, ref oMissing, ref oMissing, ref oMissing, ref oMissing, ref oMissing, ref oMissing, ref oMissing, ref oMissing, ref oMissing, ref oMissing, ref oMissing);
                object fileName2 = @"D:\Test\Combo2.doc";
                object fileFormat = WdSaveFormat.wdFormatDocument97;
                oDoc.SaveAs(ref fileName2, ref fileFormat, ref oMissing, ref oMissing, ref oMissing, ref oMissing, ref oMissing, ref oMissing, ref oMissing, ref oMissing, ref oMissing, ref oMissing, ref oMissing, ref oMissing, ref oMissing, ref oMissing);
                oDoc.Close(ref oMissing, ref oMissing, ref oMissing);
                oWord = null;
                Console.WriteLine("Done");
            }
            catch (Exception ex)
            {
                Console.WriteLine(ex.ToString());
            }
            Console.Read(); 
        }
    }
}