I have the same problem. I read that the Outlook 2003 files are in a new Unicode format and that pst files prior to Outlook 2003 are not compatible (ANSI format).
Microsoft say that the conversion cannot be handled automatically and that you have to create a new object (folder) in Outlook 2003 to import the old files into so that they become Unicode compatible. The process is described in the help section of Outlook 2003.
However it didn't work for me. I have just discovered that when you dod a repair on Outlook 2000 folders the files are placed elsewhere so I think this is likely my problem.