your code at line 1 is:
<?php session_start(); session_register("sessUsername"); $sessUsername = $HTTP_POST_VARS['txtFirstName']; ?>
you are attempting to use a post value when none is present:
$sessUsername = $HTTP_POST_VARS['txtFirstName']
the error is saying that there is no POST variable named "txtFirstName" when the page loads.
1) $HTTP_POST_VARS is deprecated in PHP 5. in most cases, you can just use $_POST
2) Any time you reference a variable that may not exist at run time, you should use an if isset statement or you will run into Undefined index errors when the variable is not set:
if(isset($_POST['txtFirstName'])) $sessUsername = $_POST['txtFirstName'];