How do I parse XML from a serial port without using files?

Unless I'm missing something here, I'd have thought that you should be able to create an XmlDocument and then use its LoadXml method to load the XML string you've received from the serial device without needing to save it to a file first.

You can then create an XPathNavigator by calling the XmlDocument's CreateNavigator method.

So, in code, that would be be something like this:

XmlDocument doc = new XmlDocument();
doc.LoadXml (xmlString);
XPathNavigator nav = doc.CreateNavigator();

Alternatively, you could forget about XPathNavigator and use LINQ to XML which can certainly parse in-memory XML strings.