Microsoft Office 2007 files format such as docx (word), xlsx (excel), pptx (power point) by default can not be open and edit with ms Office 2003 or Office XP, you need to install Office Compatibility Pack to do that. By download and installing the Compatibility Pack for Microsoft Office 2000, Office XP, or Office 2003, you will be able to open, edit, and save Office 2007 new files format.
Microsoft Office Compatibility Pack for Office 2000, 2003 and XP is free to download from microsoft site, after downloading run the exe file (FileFormatConverters.exe) and follow the instructions to complete the installation.
Now you can open, edit and save the office 2007 files on your old Office program.
Recomended see Compatibility Pack Knowledge Base article 924074 before installing.
Download Microsoft Office Compatibility Pack for Office 2000, 2003 and Office XP – direct link