If the PST file is in Unicode format then you need to use Outlook to copy it
to or export it as the older ANSI format for Outlook 2000 to be able to open
it.
You can't "load" an Outlook 2003 backup file into Outlook 2000 if that file
is in the Outlook 2003 format. In Outlook 2003, you could create a new .pst
file in the older format and copy the wanted data to it. If you can no
longer access Outlook 2003, the backup is useless.