As TCS says, the Outlook 2003 files are different to earlier versions.
It's a bit of a drastic suggestion when one is talking about MS software but you could try uninstalling Outlook 2003, installing your earlier version of Outlook, importing the .pst file and then upgrading to Outlook 2003.
If it is down to file formats that will probably do the trick ... but it's not without its potential glitches! If you go for it, I would recommend backing up your system beforehand.
Hope you get your stuff back.