Microsoft Office 2007 for PC and 2008 for Mac introduced new file formats, called "Microsoft Open XML."  These are the file formats with the "x" extensions, such as ".docx", ".xlsx", and ".pptx".

The new file formats offer the following benefits:

• Compact files
• Improved damaged-file recovery
• Better privacy and more control over personal information
• Easier detection of documents that contain macros

At this point the previous versions of Word and Excel (2003 and 2004) should all have been upgraded to be able to read the new file types, so there is no reason to continue to use the old file formats. If you're still saving to ".doc" and ".xls", here's what you can do to start saving to the Microsoft Open XML file formats.

Setting the Default "Save" File Format

Microsoft Office 2007
Open each Office 2007 application and perform the following steps on each application

1. Click the circular Microsoft Office button in the upper left corner of your screen.
2. At the bottom right of the menu, click "Word Options".
3. In the left side of the box that appears, click "Save".
4. To the right of "Save files in this format", pull down the list of file formats and select "Microsoft Word Document (*.docx)" or "Microsoft Excel Worksheet (*.xlsx)".

Microsoft Office 2008 for Mac
Open each Office 2008 application and perform the following steps on each application.

1. On the Application menu (between the blue apple and File, named "Word", "Excel", etc.) click "Preferences"
2. Under "Output and Sharing", click "Save".
3. On the "Save Word files as" dropdown menu, click the file format, Word  (.docx) and click "OK".

The next time you click save in the Office applications for which you have changed the default file format it will save in the new file formats.