Read xml file : DTD must be defined before the document root element

Hello,
On a Windows Vista Home Premium 2007, when I read a simple. Xml file without DTD via VB.Net (DataSet.ReadXml), I get the error:

"DTD must be defined before the document root element." (and the message says Line, the last character position of the file).

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But this code works for several years, including VISTA Pro (I have no machine Family on hand to test, and I don't know if it works on other VISTA Home).
Also when I add the DTD at the beginning of the file. Xml, I get the error:

"Cannot have multiple DTDs." (Line and indicates the last character position of the file)

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This behavior is it related to Vista Home, or configuration problem in Windows ... ?
Thank you for your help,
G