Using a string variable from a text box to search an Xml file using Xpath

I'm new to using XPath and would appreciate any assistance. I want to search an XML file similar to the one below using a variable entered in a text box that is part of a login page. The variable is to be used to validate the correct user. I am having difficulty coming up with the correct expression to do this. Below is an example of a sample Xml file that I have been using.I am programming in Visual Basic.I'm new to using XPath and would appreciate any