In Office 2007 Microsoft introduced . docx, . xlsx, . pptx for Word, Excel, Power Point respectively. These Office 2007 files formats can’t open in previous version like Office 2003 by default.
But Microsoft provide the solution to open docx, xlsx and pptx files in Office 2003 by installing the Microsoft Office Compatibility Pack for Word, Excel, and PowerPoint 2007 File Formats. You can Open, edit, and save documents, workbooks, and presentations in the file formats new to Microsoft Office Word, Excel, and PowerPoint 2007.
By installing above Compatibility pack you can open .docx file in Office 2003, open .xlsx file in Office 2003, open .pptX file in Office 2003. Check more options for opening Office 2007 files in Office 2000, XP versions on compatibility download page.
Downloading and Installation Screenshots
Installing compatibility pack