The Microsoft Office Suite is currently not supporting the Open Format (ODF), a file format for spreadsheets, charts, presentations and word processing documents. Most users will probably have come in contact with the Open Format in Open Office which fully supports the format.
The most common file names are .odf for word processing documents and .ods for spreadsheets. If you encounter such a file and want to open it in the Microsoft Office Suite you will notice that it is not possible. Instead of downloading another Office suite you could instead install the ODF addins for the Microsoft Office Suite which add support for the office software programs Microsoft Word, Microsoft Excel and Microsoft Powerpoint. This means support for word processing, spreadsheets and presentations, and the file formats .odt (documents), .ods (spreadsheets) and .odp (presentations).
After installing the Open XML / ODF Translator addins in the Microsoft Office Suite users can not only open but also save Open Document Formats. Supported are the Microsoft Office Suites Office 2007, Office 2003 and Office XP.
Update: The Open XML / ODF Translator Add-ins for Office have been updated recently. The biggest change is support for Microsoft Office 2010 which previously was not supported by the add-ins. It basically means that Office 2010 users can download and install the add-ins for their program to open ODF file types with it.
Other changes include a series of fixes and improvements for specific features and their transition. Installation should be straightforward. Just download the ODF add-in from the Sourceforge website and execute it on the computer that you want to use it on.
If installation is successful, you should be able to open an ".ODF" file from Microsoft Office (see menu"File>ODF...->Open ODF"). Also, when right-clicking on a ".ODF" file in explorer, you should have the option to open the document in Microsoft Office (menu "Open with Microsoft Office").
The program adds capabilities both to the file menus in Microsoft Office and to Windows Explorer's Open dialog.