function checkUserName(userName, callBack)
{
  // Build the URL to connect to
  var url = "UserIDExists.aspx?newUserName=" + escape(userName);

  // Open a connection to the server
  xmlHttp.open("GET", url, true);

  // Setup a function for the server to run when it's done
  xmlHttp.onreadystatechange = callBack;
  
  // Send the request
  xmlHttp.send();

}