XElement xml = new XElement("user",
                    new XElement("Authenticated","Yes"))
                );
xml.Save(savePath);
It works for .net 3 and above, but
 You can use XmlDocument for later versions 
    XmlDocument xmlDoc = new XmlDocument();
    // Write down the XML declaration
    XmlDeclaration xmlDeclaration = xmlDoc.CreateXmlDeclaration("1.0","utf-8",null);
    // Create the root element
    XmlElement rootNode  = xmlDoc.CreateElement("user");
    xmlDoc.InsertBefore(xmlDeclaration, xmlDoc.DocumentElement); 
    xmlDoc.AppendChild(rootNode);
    // Create the required nodes
    XmlElement mainNode  = xmlDoc.CreateElement("Authenticated");
    XmlText yesText= xmlDoc.CreateTextNode("Yes");
    mainNode.AppendChild(yesText);
    rootNode.AppendChild(mainNode);
    xmlDoc.Save(savePath);
You can use XmlWriter too as suggests @marc_s or at least you can store xml to the file like sting
using(StreamWriter sw = new StreamWriter(savePath))
{
sw.Write(string.Format("<?xml version=\"1.0\" encoding=\"utf-8\" ?>
<user><Authenticated>{0}</Authenticated></user>","Yes"));
}