“SYMPTOMS
When you use a version of Outlook that is earlier than Microsoft Office Outlook 2003, you may experience the following problem. If you to try to open or import a personal folder file (.pst) that contains information that was exported from Microsoft Office Outlook 2003, you may receive the following error message:

The file file_name.pst is not compatible with this version of the Personal Folders information service.
Contact your Administrator.
CAUSE
This problem occurs when the information in the .pst file was exported from Outlook 2003 by using the Import and Export Wizard. Outlook 2003 uses the Unicode format to export information to a .pst file….Earlier versions of Outlook can open only .pst files that are formatted in American National Standards Institute (ANSI) format.
WORKAROUND
To work around this problem, copy the contents of the Outlook 2003 .pst file to a .pst file that has not been converted to the Outlook 2003 Unicode format.”