opening a Word mailmerge datasource

I have created a mail merge datasource and open it via the Word object library in my app. If this is done with a PC with office 2000 on, then everthing is Ok. If it is a PC with office97 on, then I get an error opening data source. If I do this manually through word, I get the same problem, but, if I tick the "Select Method" check box, and select text .txt, then Word opens the datasource ok. I need to know how I do this via VB. I am opening the datasource like this:

With wdDoc.MailMerge
.MainDocumentType = wdFormLetters
.OpenDataSource Name:=MERGE_SOURCE, _
Format:=wdOpenFormatText, _
End With

If I specify a format of wdOpenFormatAuto, then I get the same error.

ANY HELP PLEASE ??? !!!!