See if this helps.
Public Class Form1
Private myFolder As String = Environment.GetFolderPath(Environment.SpecialFolder.Desktop) & "\"
Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
Dim xmlDoc As New Xml.XmlDocument, ndKeyList, ndStringList As Xml.XmlNodeList, iNodeIndex As Integer
With xmlDoc '// load File and lists.
.Load(myFolder & "test.xml") : ndKeyList = .GetElementsByTagName("key") : ndStringList = .GetElementsByTagName("string")
End With
For i As Integer = 0 To ndKeyList.Count - 1 '// loop thru key list.
If ndKeyList(i).InnerText = "Serial Number" Then
iNodeIndex = i '// set the Index where located.
Exit For '// exit.loop since done.
End If
Next
MsgBox(ndStringList(iNodeIndex).InnerText) '// display string at the same Index as the key.
End Sub
End Class