New Microsoft Office file formats - .docx .pptx .xlsx
So, Microsoft has finally done something right! 
Office 2007 uses the OpenXML file format that has received ECMA approval (that's a good thing for interoperability).
The biggest change is that the file itself is actually a collection of other files within--go ahead and change the extension of one to .zip and open it up. You'll see a hierarchy of XML and image files that make up the document, presentation, or spreadsheet. This is great news, as it allows damaged documents to be troubleshot without knowing Office's internal file structure. However, in making the switch, it breaks compatibility with old versions. Or does it?
IMPORTANT: Before installing the compatibility pack, please go to http://update.microsoft.com and make sure all of the latest Office updates are installed--especially critical updates.
Microsoft has released compatibility packs for older versions of Office (XP and 2003) for the PC here:
http://www.microsoft.com/downloads/details.aspx?FamilyId=941b3470-3ae9-4aee-8f43-c6bb74cd1466&displaylang=en
and for the Mac here:
http://www.microsoft.com/mac/downloads.aspx?pid=download&location=/mac/download/office2004/converterbeta_0_2.xml
Install these, and for the most part, you should be able to open and print files made in the new version.
If you're already using Office 2007, you can set the program to continue to save files in the older format by pulling down the list of file formats under the file name on the Save dialog box and choosing the correct version.