XPathQuery is a tiny application I developed several years ago for my students, so that they can play with some basic XML searches using XPath, and transform XML using XSLT. It has minimal requirements (just .Net 2.0) and has a very simple user interface.
To start with, enter some XML code or just open an XML file from the toolbar (when using the Open button, ensure that you have the XML panel selected, as it behaves differently according to the selected pane).
If you want to test an XPath query, enter it in the top textbox and hit Enter: the resulting (selected) XML fragment will be shown in the Results pane. Should your XPath query syntax be incorrect, the textbox background will turn red.
To test an XSLT transformation, load (or type) XSLT code by clicking the Open button when the XSLT panel is selected. Then click the Transform button and see the output result. If the output is HTML, you can directly see its rendering in the HTML panel.
Should you need to define custom namespaces, just enter them in the Namespaces panel. Finally, you can change the transform output encoding with the top dropdown list (defaulting to UTF-8).