Office 2007 utilizes yet another file format as its default, creating documents with the extension .docx in Word, .xlsx in Excel, etc.

To change the default setting so that files created are automatically saved in a format that can be read by users with Office XP or Office 2003, open the appropriate program, click the Office Button (that round guy in the upper left corner of the screen), and then click on Word (or other program) Options in the lower right side of the menu box displayed. One of the choices on the left-hand navigation bar is Save. Select this option and you will see the combo (drop-down) box that lets you specify the compatible format as your default.