Since the introduction of Office 2007 many people have had issues with compatibility with Office 2003 and older versions of Office. The issue is that Office 2007 by default saves documents in its new format, unless you tell it not to. This new format is not compatible with the older versions of Office.
To get around this issue Microsoft have produced a compatibility pack which will convert the Office 2007 files to Office 2003 format for you. So if you still have Office 2003 simply go here Microsoft Compatibility Pack and download the file. Then install it and it will do the rest.