You will need to add a reference to word (assuming you have this on your machine, otherwise it will become very complicated)
Then simply
'create a new instance of word, and a placeholder for the document
Dim wrdApp As New Word.Application
Dim wrdDoc As Word.Document
'open the document you want
Set wrdDoc = wrdApp.Documents.Open("D:\MyDoc.doc", True)
'save the document specifying the format
wrdDoc.SaveAs "c:\mydoc.txt", wdFormatText
'dont forget to tidy up
wrdDoc.Close
Hope this is ok.