function submit_form() {
  /*** Check for errors ***/
  error_message = '';
  form = document.forms['contact'];
  /* Name */
  if (form.elements["name"].value == '')
    error_message += "Please enter Company/Name.<br />\n";
  /* Zip code */
  if (form.elements["zip"].value == '')
    error_message += "Please enter Zip Code.<br />\n";
  /* Phone/Email */
  if (form.elements["phone"].value == '' && form.elements["email"].value == '')
    error_message += "Please enter either a phone number or an email address (or both).<br />\n";
  else if (form.elements["Phone"] && form.elements["Phone"].checked && form.elements["phone"].value == '')
    error_message += "Oops! You've asked to be contacted via phone, but haven't entered a phone number!<br />\n";
  /* Fax */
  if (form.elements["Fax"] && form.elements["Fax"].checked && form.elements["fax"].value == '')
    error_message += "Oops! You've asked to be contacted via fax, but haven't entered a fax number!<br />\n";
  /* Address */
  if (form.elements["Mail"] && form.elements["Mail"].checked && (form.elements["address"].value == '' || form.elements["city_state"].value == ''))
    error_message += "Oops! You've asked to be contacted via mail, but haven't entered a full address!<br />\n";

  /*** Display Errors ***/
  if (error_message != '') {
    document.getElementById('error').innerHTML = error_message;
  /*** Submit Form! ***/
  } else {
    form.elements["js"].value = '1';
    form.submit();
  }
}
<!-- cached -->