Versions of PST Files
Here is a description of the PST files from both Outlook 97 and Outlook 2003;
Outlook 97
Starting with Microsoft Outlook 97 the Personal Storage Table file is encoded in ANSI format. It has a maximum file size of 2 GB. If the file exceeds this size it will become corrupted. This format is supported by Microsoft Outlook 97, Outlook 98, Outlook 2000, Outlook 2002, and Outlook 2003 Internet Message Access Protocol Version 4rev1 (IMAP4) accounts and HTTP accounts.
Outlook 2003
Starting with Outlook 2003 the Personal Storage file is encoded in unicode format. It allows a user-definable maximum file size up to 33TB that exceeds the previous limit of 2GB. This format is supported by Microsoft Outlook 2003 and Outlook 2007. A file that is created in the personal folders format in Outlook 2003 or Microsoft Office Outlook 2007 is not compatible with earlier versions of Microsoft Outlook and cannot be opened by using those older versions.