You may try the the following measures:
i) Export all data from Outlook 2003 to a .pst file and then import it to Microsoft Outlook 2007 (if you are using POP3/IMAP)
ii) If you are using Microsoft Exchange server then data will remain in your Exchange mailbox, and you have to configure a new MAPI profile in Outlokk 2007 (although Autodiscover option in outlook should do this for you)
Hope it would hep you!