function checkForm()
{
   // the variables below are assigned to each
   // form input
   var gname, gemail, gurl, gmessage;

   with(window.document.guestform)
   {
      gname    = txtName;
      gemail   = txtEmail;
      gurl     = txtUrl;
      gmessage = mtxMessage;
   }

   // if name is empty alert the visitor
   if(trim(gname.value) == '')
   {
      alert('Please enter your name');
      gname.focus();
      return false;
   }
   // alert the visitor if email is empty or
   // if the format is not correct
   else if(trim(gemail.value) != '' && !isEmail(trim(gemail.value)))
   {
      alert('Please enter a valid email address or leave it blank');
      gemail.focus();
      return false;
   }
   // alert the visitor if message is empty
   else if(trim(gmessage.value) == '')
   {
      alert('Please enter your message');
      gmessage.focus();
      return false;
   }
   else
   {
      // when all input are correct
      // return true so the form will submit
      return true;
   }
}

/*
Strip whitespace from the beginning and end of a string
*/
function trim(str)
{
   return str.replace(/^\s+|\s+$/g,'');
}