how to append child node to an existing xml file

This is my code to create xmlfile
when i'm writing into it for first time it's working fine.when i'm inserting the content again it showing error only one root node is allowed.
The problem is when i'm additing nodes again i need to append it as a child node to the root node but i don't know how to do it.
Please help me to do this

Sub CreateXmlFile()
       Dim objReportNodes As New System.Text.StringBuilder()
       Dim strDetails As String
       strDate = Request.Form("tBDate").ToString

       strTime = Request.Form("lBTime")

       strSubject = Request.Form("txtsubject")
       strAjanta = Request.Form("txtAjanta")
       strDetails = "<?xml version='1.0' encoding='windows-1252' ?>"
       strDetails &= "<Messages><Date>" & strDate & "</Date>" & _
                     "<Time>" & strTime & "</Time>" & _
                     "<Subject>" & strSubject & "</Subject>" & _
                     "<Ajanta>" & strAjanta & "</Ajanta>" & _
                     "</Messages>"
       objReportNodes.Append(strDetails)

       Dim objFile As System.IO.TextWriter = System.IO.File.AppendText("D:\XMLFile3.xml")

       objFile.WriteLine(objReportNodes)
       objFile.Close()
       objFile = Nothing
       objReportNodes = Nothing
       Call ReadXmlFile()
   End Sub

i tried using AppendChildNode() but here i'm using textwriter so it is not converting to system.xml.xmlnode

Tags: Visual Basic, XML