In September, we added the ability to export Google documents to the
newer Microsoft Office formats that rely on open standards (.docx,
.xlsx, .pptx). To continue adding new features and innovations to Google
Drive and Docs, we’ll be focusing our resources on exporting to these
newer open formats rather than the older, proprietary Office formats
(.doc, .xls, .ppt) that were used in Office 97-2003.
As we announced in October, after January 31, 2013, users will not be
able to export files to the older Office 97-2003 formats. For users who
still use Office 97-2003, we recommend installing the free
compatibility plugin from Microsoft, which will allow them to open the modern Office file types.
For more information
http://googleappsupdates.blogspot.com.au/2012/10/docs-export-format-change-delayed-to.html
http://www.microsoft.com/en-us/download/details.aspx?id=3
