From stream to (xml) string problem with chars\encoding

Hi,
i getting stream message (from biztalk) in this case, convert it to string and load it to xmldocument.
im getting data at root level is invalid.
when i debug the code, i can see there are chars (???? question marks) beside the xml like:

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Invalid at the top level of the document. Error processing resource 'file:///C:/Documents and Settings/eladro/Local Setting...

??<
^
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how it can be solved??
thanks.

this is my simple code:
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IBaseMessagePart bodyPart = inmsg.BodyPart;
System.IO.Stream originalStrm = bodyPart.GetOriginalDataStream();

byte[] buffer = new byte[originalStrm.Length];
originalStrm.Read(buffer, 0, Convert.ToInt32(originalStrm.Length));
XMLData = System.Text.ASCIIEncoding.ASCII.GetString(buffer);

XmlDocument xmldoc = new XmlDocument();
xmldoc.LoadXml(XMLData);
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