A lot of people are still using Office 2003, and having trouble working with customers and vendors who have upgraded to Office 2007.  Microsoft has an applet you can install that will allow you to work with those files seamlessly, which you can find here:

Microsoft Office Compatibility Pack

If your co-workers, or other people you share files with, are still using Office 2003, don’t forget to make sure your Office applications save as 2003 by default.

• For Word, that’ll be under the Edit menu–choose Preferences, then the Save tab, and choose Word 2003 under the Default Format section.
• For Excel, it’s the Tools menu, then choose Options, and the Save tab.