XML transforms in the .net framework changed from 1.0 to 1.1. When you compile your code for 1.1 the compiler throws

'System.Xml.Xsl.XslTransform.Transform(System.Xml.XPath.IXPathNavigable, System.Xml.Xsl.XsltArgumentList, System.IO.TextWriter)' is obsolete: 'You should pass XmlResolver to Transform() method'

This article shows you how to change your transforms and why you should.