docx is actually zip!

Source: http://labnol.blogspot.com/2007/07/edit-print-or-convert-docx-to-doc.html

The DOCX files, which are now the default file extension in Word 2007, are essentially a bunch of zipped XML documents - [you can even rename the extension from .docx to .zip and extract the contents using Winzip]

Ed: And this is true for PPT as well - try renaming ".pptx" to ".zip" and open the files.

Alternatively, you can convert .docx to .doc or extract text from .docx files online using the free Zamzar.com or docxtodoc.com service. Unlike the Microsoft compatibility pack, these docx to doc conversion services do not require copy of Microsoft Office.

It is also possible to open and edit a .docx file in OpenOffice or StarOffice using the free Open XML Translator from SourceForge which converts .docx Word documents to .odf and vice versa.