Outlook 2003 saves its data in so called "Personal Folder Files" .pst, which is an acryonym for "Personal Storage Table."
The .pst personal folder file is stored in a folder of your Windows profile. If you don't change the path manually, the standard path on a Windows XP installation is
C:\Documents and Settings\PROFILE_NAME\Local Settings\Application Data\Microsoft\Outlook\
One important advice: if the user had "auto archive" enabled, there could be another (or more) .pst in your backup:
archive.pst. The file contains objects which were exported in order to keep the main .pst file from growing too large. Search for all *.pst files.