Messy code while modifying the xml file with installscirpt

Question

We want to modify the node of an xml file such as web.config and we get messy code after saving the file. The code we use is below:

   set  xmlDocument = CoCreateObject("Msxml2.DOMDocument.4.0");
   if (!IsObject(xmlDocument)) then
      return -1;
   endif;

   xmlDocument.async=FALSE; 
   xmlDocument.setProperty("SelectionLanguage","XPath");
   xmlDocument.load(xmlFileName);
   if(IsObject(xmlDocument)=FALSE) then
      return -1;       
   endif;
   set xmlNode = xmlDocument.selectSingleNode(nodePath);
   if(IsObject(xmlNode)=TRUE) then
   xmlNode.Attributes.getNamedItem("value").nodeValue = value;
   xmlDocument.save(xmlFileName);

I tried to use many ways to resolve this problem, such as BOM or Encoding in xml file head. But I haven't found the right way.

Now I would like to know two things:

1. the modifed file encoding.
2. modify the file with its encoding.

who can help me?