Microsoft Office 2007 uses new formats to save files in Word (.docx), Excel (.xlsx) and PowerPoint (.pptx). Previous versions of Office can't open these formats without installing the Microsoft Office Compatibility Pack in addition to Microsoft Office 2000, Office XP, or Office 2003. The Compatibility Pack can also be used in conjunction with the Microsoft Office Word Viewer 2003, Excel Viewer 2003, and PowerPoint Viewer 2003 to view files saved in these new formats.

If you have Office 2007 and wish to allow your documents to be opened by previous versions without the Compatibility Pack, then when you save each document, click 'Save as' and select 'Word 97-2003 Document'. You can set each program to save in this format by default by going to the 'Office Button' in Word, Excel or PowerPoint 2007 (the round thing at the very top left), then click on 'Word Options' > 'Save' (or equivalent), then where it says 'Save files in this format:' use the drop-down menu to select the '97-2003 Document' format.