Updating a Xml file

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C#

I have never worked with config files before but now I need to learn fast, so please bear with me if I'm making obvious mistakes or asking silly questions. I try to add new nodes to the config file and hard code their values with the following code;

string pathToConfig = Directory.GetCurrentDirectory();

            // check for the config file
            if (File.Exists(string.Format(@"{0}\config.xml", pathToConfig)))
            {
                Console.WriteLine("Found the config.xml file, reading..");

                // load the xml file
                XmlDocument xmlDoc = new XmlDocument();
                XmlTextReader xmlReader = new XmlTextReader(string.Format(@"{0}\config.xml", pathToConfig));
                xmlDoc.Load(xmlReader);
                XmlElement elem = xmlDoc.CreateElement("XPos");
                XmlText text = xmlDoc.CreateTextNode("400");
                xmlDoc.DocumentElement.AppendChild(elem);
                xmlDoc.DocumentElement.LastChild.AppendChild(text);

                Console.WriteLine("Display the modified XML...");
                xmlDoc.Save("config.xml");
                XmlNodeList nodeList = xmlDoc.ChildNodes[2].ChildNodes[0].ChildNodes;
                label15.Text = nodeList[0].InnerText.ToString();
            }

It reads data from the xml document with no problems, however when it gets to the save line it gives an error saying the process cannot access the file because it is being used by another process. The file is located in my debug folder and I don't have it open. I know the node is being added because when I save to the console I can see the updated xml text. Any help will be greatly appreciated.