Office 2007 Compatibility Pack
If you have received a file recently that was produced using Office 2007 (Word, Excel, etc) you may not have been able to open it.
Download this “Compatibility Pack” from Microsoft. That will allow you to open and edit Office 2007 files without the need for you to upgrade.
Here is a pretty good explanation, by Jack Schofield, of “why” Microsoft is using .docx as their file extension.