"<a>
    <b>zyx</b>
    <b>wvu</b>
    <b>tsr</b>
    <b>qpo</b>
</a>"

Dim xmlSearchText As New XmlDocument()
xmlSearchText.LoadXml(SearchTextBox.Text)

Dim selectedNode As XmlNode = xmlSearchText.SelectSingleNode(txtSearch.Text)

a/b[.='tsr']

selectedNodenode = xmlSearchText.SelectSingleNode("a/b[.='tsr']");
selectedNode.ParentNode.OuterXml.IndexOf(selectedNode.OuterXml)

XPathDocument document = new XPathDocument("file.xml");
XPathNavigator navigator = document.CreateNavigator();
XPathNavigator node = navigator.Select("a/b[.='tsr']");
IXmlLineInfo info = ((IXmlLineInfo)node);
Console.WriteLine("Found at ({0},{1})",info.LineNumber,info.LinePosition);