underneath Office 2007 documents
just rename any M$ office 2007 document to .zip and open it in zip program you will find what inside has i.e. xml documents, . all office 2007 documents are actually zip files with different extensions, I verified the PK header too. Thanks Shafquat for sharing this ...
http://msdn2.microsoft.com/en-us/library/aa338205.aspx
http://www.ghacks.net/2007/07/25/how-do-i-open-docx-files/
http://help.lockergnome.com/office/docx-zip-ftopict981566.html
Cheers
Nasir Ali Khan