The easiest is to either use the pst you used with outlook 2003 or open it
in outlook 2007 (file, open, outlook data file) and move or copy the
contacts into the new pst.
Simplest is with Outlook closed to open the Mail applet in Control Panel,
click Data Files, Add the original Outlook 2003 data file, set it as the
delivery location, remove any other PSTs (except, perhaps, an Archive PST),
then start Outlook. Everything will be exactly as it was in Outlook 2003.
You may have to redefine your mail accounts.
When transferring data from one Outlook to another, never use export/import.