Xpath

 Name space :         using System.Xml;
                                 using System.Xml.XPath;

 Xpath objects

                 1. XPathNavigator nav;to evaluate XPath expressions
                 2. XPathDocument docNav; - To hold the xml document
                 3. XPathNodeIterator NodeIter;- its  iterate through selected nodes

 // Open the XML.
    docNav = new XPathDocument(@"c:\books.xml");

  // Create a navigator to query with XPath.
     nav = docNav.CreateNavigator();
Reference :
http://support.microsoft.com/kb/308333