Outlook: Shared PST files?

At my office we want to be able to create a Pst inbox for every new job we get, and have all correspondence moved into it. The theory works fine for one person, but if two people try to open the same PST file outlook chucks a wobbly and doesn’t allow it.

Is there any other way we can do this, or if not is there any third party software that can make a shareable outlook pst file?

We are using Server 2003 and Outlook 2003.

Thanks for any help.