Web service to Upload XML string to DB : gets error

I have a web service that accepts the xml string and uploads to the database..

  [WebMethod]
        public void Service(string XML)
        {
            try
            {
                using (DBDataContext db = new DBDataContext())
                {
                    db.ImportXML(XElement.Parse(XML));
                }

               
            }
            catch (Exception err)
            {
               
                throw err;
            }
        }

iam getting the below error
HTTP Transport error javax.xml.soap.SOAPException Error parsing envelope: (77,40) Invalid char in Text.

i know it is occurning beceause of some invalid characters in the xml string..
is there any other way to do it.. can anyone help.

also what do i need to do if i need to upload 100mb data..