Use xlinq:
var x = new XElement("Root",
new XElement("PC", new XAttribute("ID", "1")),
new XElement("PC", new XAttribute("ID", "6")),
new XElement("PC", new XAttribute("ID", "12")),
new XElement("PC", new XAttribute("ID", "28")));
x.Add(new XElement("PC", new XAttribute("ID", "36")));
Console.WriteLine(x.ToString());