In view of the query put forward, we would like you to refer the following technote link which would give you a detail explaination of items that have to be backed up while considering Backups of Exchange 2003.
1) Title:- Is backing up Exchange Mailboxes equivalent to backing up the Exchange Information Store?
http://seer.support.veritas.com/docs/231250.htm
2) Title:- How to protect Exchange 2000 and 2003 servers using the Backup Exec for Windows Servers Exchange Agent
http://seer.support.veritas.com/docs/266074.htm
We hope this helps.