In Office 2008 for Macs (and 2007 for Windows), Microsoft introduced a new default file format based on XML standards. These "dot-x" files cannot be opened by previous versions of Office. However, Microsoft does allow you to change your file saving format to allow users of older software to access your files. These settings can be changed as follows:

In Word:

Go to Preferences and select the "Save" icon from the second row:

Then select the pulldown menu from "Save Word files as" and choose your default file format.

By default, all Word documents you create and save will be in this format.

In Powerpoint:

Go to Preferences, select the "Save" icon from the top row, then select the pulldown menu from "Save Powerpoint files as" and choose your default file format.

By default, all Powerpoint documents you create and save will be in this format.

In Excel:

Go to Preferences and select the "Save" icon from the third row:

Then select the pulldown menu from "Save files in this format" and choose your default file format.

By default, all Excel documents you create and save will be in this format.