Google Drive’s Docs, Sheets, and Slides apps are scheduled to cease supporting exports to Microsoft’s older Office 97–2003 formats at the end of the month. According to the Google Apps blog,
In September, we added the ability to export Google documents to the newer Microsoft Office formats that rely on open standards (.docx, .xlsx, .pptx). . . . As we announced in October, after January 31, 2013, users will not be able to export files to the older Office 97-2003 formats [e.g., .doc, .xls, .ppt]. 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.