I would “restore” this backup not to original location, anywhere else on the network. then I would look into purchasing a product called Ontrack Power Controls which would be able to extract straight from an Exchange database file (EDB).
In exchange 2003 you can restore the database to a recovery storage group on the existing exchange server and then use exmerge to extract the mailbox you want from that database to a pst file. The recovery Storage group is separate from the existing storage group and restoring using that method will not cost you any additional money for anotehr program. It is a fairly simple process.