its not necessary to upgrade your office 2003 or 2002 to office 2007 just to open the files with office 2007 extensions. like *.xlsx , *.docx etc. there is a compatibility pack available on microsoft website.
http://www.microsoft.com/downloads/details.aspx?FamilyId=941B3470-3AE9-4AEE-8F43-C6BB74CD1466&displaylang=en