If you have MS Office 2003 installed on your computer, like most of us do, you may run into the problem that I did when I was emailed a file with the ".docx" extension. When I tried to open it, MS Word 2003 did not know how to open it.
It turns out that this file extension (with the trailing 'x') is a new format that is part of MS Office 2007. To open files in MS Office 2003 with the "x" (.docx, .xlsx, and .pptx), you will need to download and install the program FileFormatConverters.exe from the Microsoft website. This download will add the capability to open MS Office 2007 files from Office 2003. You might not get the full impact of some of Office 2007's new features, but you will be able to open, edit, and/or print the document.