You don't. You open it.
2003 and 2007 share the same pst format so there's no compatibility
problems.
Copy the pst to your HDD (do NOT overwrite any existing pst file). Remove
the Read-Only attribute.
Then in Outlook do File-Open-Outlook Data File and navigate to where you put
it.
You then have two options - either copy the data to your existing pst file,
or set the copied one as the default delivery location.
Locate the pst file on a suitable location on your HD, eg Documents Folder.
Start Outlook, File>Open>Data File.........browse to that location
If neccessary copy data from this set of Personal Folders to your origonal
set