It depends on if this is a Outlook 2003 and Exchange configuration. PST files, like personal archives, etc are stored on the users hard drive, completly offline and exported from the information store on the server. A OST file is stored on the server and on the person's machine. This is so people can work offline and still have access to their email, then go online when they get a chance and synchronize changes.
A crashed hard drive or virus attack depends on the situation. If the hard drive fails in regards to hard ware (head crash) it would have to be shipped to a Data Recovery Center. This could cost up to 10K. Usually the center basis the cost ammount on how much data needs to be recovered.
I've seen several hard drives that have had Master Boot Record corrpution and or a corrupt Windows install. This is easy to recover using various free utilities.
If your users are on a workplace network, you may want to look into a shared network resource to backup archive PST files and other documents.