to set a default value, change:
<input id="Full_Name" name="Full_Name" type="text" value="<?php echo((isset($_GET["invalid"])?ValidatedField("index2","Full_Name"):"")); ?>" class="formTextfield_Large" tabindex="1" onblur="hideServerError('Full_Name_ServerError');"/>
to:
<input id="Full_Name" name="Full_Name" type="text" value="<?php echo((isset($_GET["invalid"])?ValidatedField("index2","Full_Name"):"Your Default Value Here")); ?>" class="formTextfield_Large" tabindex="1" onblur="hideServerError('Full_Name_ServerError');"/>