说明：只有在xp ie 6环境下有这种问题，ie6以上版本都没问题。

代码：

using (MemoryStream ms = new MemoryStream())

{
XmlWriterSettings settings = new XmlWriterSettings();
settings.Indent = true;
settings.Encoding = new UTF8Encoding(true);

//设置换行符
settings.NewLineChars = Environment.NewLine;

using (XmlWriter xmlWriter = XmlWriter.Create(ms, settings))
{
xmlWriter.WriteStartDocument(false);
xmlWriter.WriteStartElement("contact");
xmlWriter.WriteElementString("contactID", contactID);
xmlWriter.WriteElementString("name_en", name_en);
xmlWriter.WriteElementString("name_ch", name_ch);
xmlWriter.WriteElementString("PCDSNO", PCDSNO);
xmlWriter.WriteElementString("contactQRCodeImg", contactQRCodeImg);
xmlWriter.WriteEndElement();
xmlWriter.WriteEndDocument();
}

string xml = Encoding.UTF8.GetString(ms.ToArray());
Response.Clear();
Response.ClearHeaders();
Response.Buffer = false;
Response.ContentType = "text/xml; characterset=utf-8";
Response.AppendHeader("Content-Disposition", "attachment;filename=" + HttpUtility.UrlEncode(ContactNO + @".xml"));
Response.AppendHeader("Content-Length", ms.Length.ToString());
Response.Write(xml);
Response.Flush();
Response.End();
}