Help with Replacing text in a file using a vbscript

I have the following script which I am wanting to

1. Take the prf file and read it

2. Edit a line in the  outlook .prf file replacing it with the text I have asked it to replace which is the full name of the user pulled from the currently logged on user

The problem I am seeing is that when the temp file is being generated it is throwing off the formatting of the file hence the script doesn't work. Can anyone help me please.

set objSysInfo = CreateObject("ADSystemInfo")
struser = objSysInfo.Username
set objUser = GetObject("LDAP://" & strUser)
strFullName = objUser.Get("displayName")

Const ForReading=1
Const ForWriting=2

Set objFSO = CreateObject("Scripting.FileSystemObject")
folder = "C:\test"
'filePath = folder & "file.txt"
filePath = "c:\test\test2007_3.prf"
Set myFile = objFSO.OpenTextFile(filePath, ForReading)
Set myTemp= objFSO.OpenTextFile(filePath & ".tmp", ForWriting, True)
Do While Not myFile.AtEndofStream
myLine = myFile.ReadLine
 If InStr(myLine, "MailboxName=%UserName%") Then
  myLine = "&strFullName"

 End If

 myTemp.WriteLine myLine
Loop
myFile.Close
myTemp.Close
objFSO.DeleteFile(filePath)
objFSO.MoveFile filePath& ".tmp", filePath

Christopher