Ok, then here a quick example of what you can do :
strInput = InputBox( "Computer Name:" )
Const ForReading = 1
Const ForWriting = 2
UnattendFile = "c:\windows\panther\unattend.xml"
Set objFSO = CreateObject("Scripting.FileSystemObject")
Set objFile = objFSO.OpenTextFile(UnattendFile, ForReading)
strText = objFile.ReadAll
objFile.Close
strNewText = Replace(strText, "<ComputerName>%ldHostname%</ComputerName>", "<ComputerName>"& strInput &"</ComputerName>")
Set objFile = objFSO.OpenTextFile(UnattendFile, ForWriting)
objFile.WriteLine strNewText
objFile.Close
It might need some changes, it depends of your unattend.xml and it's designed to work with WSCRIPT, not CSCRIPT.