You can do this a number of ways.
Using XmlDocument and XmlNode etc.
http://www.stardeveloper.com/articles/display.html?article=2009030701&page=1[
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Using Xml Linq....e.g.
XDocument posLog = XDocument.Load(posLogPath);
XNamespace ns = "http://www.somenamespace.org/IXRetail/namespace/";
var transactions = from transaction in posLog.Descendants(ns + "Transaction")
where transaction.Descendants(ns + "SomeRequiredCriteria").Any()
Try google, there are lots of examples of both techniques