Possibly, by making sure you start your script with a [man]session_start()[/man] command, you will remove the problem, since it is the automatic session_start() done by session_register() if it has not yet been done that is ultimately causing the error. However, since using session_register is more-or-less deprecated, you could change your script to do a session_start() at the very beginning, then assign session values to the $_SESSION array instead of using session_register().
The next option, if you don't want to do that, is rearrange the code logic and/or remove any inadvertent output prior to that call to session_register(). (Remember that any text -- including spaces or newlines -- outside of <?php...?> tags is output.)
The last option would be to start your main script with a [man]ob_start()[/man] command to have your script do output buffering.
