Most programs, such as Backup Exec, have two different kinds of backups for Exchange.
The first is a backup of each database. The second would be a "brick level" backup. These are setup as different backup jobs, and can be ran at the same time. The bricklevel backup job would back up each mailbox individually, and takes a he!! of a long time usually. This is why most people using tape limits this to important people, or don't use it at all.
If you are not doing a brick level backup, you cannot simple restore one mailbox.
If you are using Exchange 2003, the best place to start on your quest of restoring on person's mailbox would be the below link:
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