I don't think there's a difference in the format, just install outlook 2003 and it will create an outlook.pst, either in:
x:\Documents and Settings\<Username>\Local Settings\Application Data\Microsoft\Outlook
or x:\Documents and Settings\<Username>\Application Data\Microsoft\Outlook
Exit outllok and make sure the process "outlook" is not running. Rename the new outlook.pst file as outlook.bak then copy your old ooutlook.pst file in place of the newly created outlook.pst file.
Start up outlook and things should be fine....