C# Office 2003 Interop code stops with AccessViolationException - but didn't before

This problem perplexes me. It's a program which was working fine, then the same code stopped working.
Something must have been 'updated' since.

It's a C# WinForms application being developed on VS 2010 under Vista.
It uses MS Office 2003 + Interop Libraries (ver. 11) (All Copy Local)

It is supposed to open a Word document, make changes to the text and then save the document to another location.

Code: -

using System;
using System.Collections;
using System.Collections.Generic;
using System.Text;
using System.Windows.Forms;
using Microsoft.Office.Interop.Word;

namespace Shared {
   class WordSession {
      public void Test(string FileName, string OldText, string NewText) {
         object X = System.Type.Missing;
         ApplicationClass _Word = new Microsoft.Office.Interop.Word.ApplicationClass();
         object DocumentName = FileName;
         object IsVisible = true;
         object IsReadOnly = true;
         object Format = WdOpenFormat.wdOpenFormatAuto;
         Document _Document = _Word.Documents.Open(ref DocumentName, ref X, ref IsReadOnly, ref X, ref X, ref X, ref X
               , ref X, ref X, ref Format, ref X, ref IsVisible, ref X, ref X, ref X, ref X);
         _Document.Activate();
         _Word.Visible = true;
         this.Document.SelectAllEditableRanges(ref _Missing);
         object Source = OldText;
         object Target = NewText;

       // This line fails with the AccessViolationException
         _Word.Selection.Find.Execute(ref Source, ref X, ref X, ref X, ref X, ref X, ref X, ref X, ref X
               , ref Target, ref wdReplaceAll, ref X, ref X, ref X, ref X);
      }
   }

This code used to work - in fact its released and live.
Now its stopped working on my development machine so I figure there's an obscure setting unset somewhere.
Anyone got any ideas?
Much appreciated.