Just to clarify
Are you talking about your outlook .pst files used with Outlook 2007?
If so, using the import export function in Outlook (either 2003 or 2007) will import .pst files created in 2007.
The correct way would be:
Open the Outlook 2007.
Go File then Import Export
and Export as a .pst
Save it to an easily accessible location (i.e Desktop or a USB if it's on another PC)
Go to Outlook 2003
File
Import Export
Then import the .pst
Lots of help here:
https://answers.yahoo.com/question/index?qid=20140704025027AA0onZj