XPath; and then came XQuery
The XPathNavigator is recommended for methods that accept XML input or return XML as output. .NET 1.x supports a read-only cursor over XML data sources. Though this is about to change in .NET 2.0 with the XPathEditableNavigator in sight. Dare says XQuery is the future and already incorporates pretty much everything what makes XPath 2.0 such interesting technology.
If today has your focus I suggest you give Praj Joshi’s XPath Visualizer a spin; otherwise invest in XQuery.