Are there any odd read write permissions in the properties of the .pst file, I assume you didn't create it with any passwording.
Outlook 2003 should be able to open any file that it creates. I cant see that Windows XP service packs should alter how Outlook operates, especially file access. It worryingly might be that your .pst file is corrupt in some way.
You may find some information if you do a google on the Outlook Newsgroup:
http://communities2.microsoft.com/co...&lang=en&cr=US