ReadXML and WriteXML

        public void BuildDataSetXML()
        {
            //sql is a placeholder variable to store the sequel statement
            string sql = String.Format("SELECT TOP * FROM [CustomerInfo] WHERE CmpID={0}", CompanyID.ToString());

            //fileNameWithAllData is a placeholder variable to store the name of the xml file 
            string fileNameWithAllData = String.Format("ReadCustomerInfoByCompanyID_{0}.xml", CompanyID.ToString());

            //process the result of the sequel statement to dsAllData variable
            DataSet dsAllData = ProcessDataset(CommandType.Text, sql);

            //Dataset is stored in the fileNameWithAllData
            dsAllData.WriteXml(fileNameWithAllData);
        }

How do I WriteXML to a location that I want such as (C:\Desktop\fileName)? Right now the xml file is saving somewhere else.

public DataSet GetCustomerInfo(int CompanyID)
     {
         //pass in the method above
         BuildDataSetXML();

         //pass in the variable from the above method
         string fileNameWithAllData = String.Format("ReadCustomerInfoByCompanyID_{0}.xml", CompanyID.ToString());

         //Instantiate ds
         DataSet ds = new DataSet();

         //Read into the fileNameWithAllData 
         ds.ReadXml(fileNameWithAllData);

         //Query the fileNameWithAllData (ReadCustomerInfoByCompanyID_{0}.xml) and only get lastName and firstName in the field
         string sqlFieldSelected = String.Format("SELECT lastName, firstName FROM fileNameWithAllData");

         //fileNameSelectedField is a placeholder variable to store the name of the xml file 
         string fileNameSelectedField = string.Format("GetName.xml").ToString();

         //process the result of the sequel statement to dsSelectedData variable
         DataSet dsSelectedData = ProcessDataset(CommandType.Text, sqlFieldSelected);

         //write out the data from sequel to XML
         ds.WriteXml(fileNameSelectedField);

         return ds;
     }

I want to read into(ReadXML) the XML file that was already created and want to write out(WriteXML) a new XML with selected fields(firstName and lastName). Thank you