You screwed up. Not your fault since you are not clairvoyant. It is
Microsoft's.
Microsoft pretends that importing/exporting is a valid way to transfer
Outlook data. They have never acknowledged the fact that it is not. It is
time for them to do so. I'm growing weary of doing it for them.
Microsoft neglected to include an option to export your Outlook 2003 data to
a file format that would be compatible with any earlier versions. They
should have addressed this deficiency long ago. They seem to have no clue as
to how end users use their program.
Until they do, your only option is to create a new PST file in "Outlook
97-2002" format from Outlook 2003 and then copy your data into that file.