If you are still using Office 2003 or an earlier Office product then you may not know that the new Office 2007 application uses a new file format that cannot be opened by earlier Office products. You may have had a client or prospect send you a document that you could not open in your version of Word, PowerPoint or Excel that ended with an X like a .docx, .pptx or .xlsx file.
Don't send your customer a note asking them to resend the file requiring them to resave it in the old Office format, just download the Office 2007 file compatibility pack from Microsoft.
You can download this compatibility pack free from Microsoft.
The new Office 2007 file format uses XML to keep file sizes incredibly small and to allow for future application interaction. Changing to the new XML file structure is a sweeping change and a controversial one as important changes always are. This compatibility pack allows users of Office 2000, Office XP, or Office 2003 to open edit, and even save files in the new XML format allowing for easy file transfer and exchange with customers already using Office 2007.
So if you aren't ready to upgrade your Office products to 2007, with the compatibility pack no one will know that you are still using Office 2000 or Office 2003.