Microsoft Corporation - (Free)
Microsoft Office Compatibility Pack makes your older versions of Microsoft Office compatible with Microsoft Office 2007.
Microsoft Office Compatibility Pack is a software add-on for Microsoft Office 2000, Office XP and Office 2003 developed to make older versions of Microsoft Office suite compatible with the 2007 or 2010 versions of files. It , XLSX and PPTX, are not supported by earlier versions of Word, Excel and PowerPoint without this program.This way, you can edit and save files in the new Word, Excel and PowerPoint 2010 file through programs under older versions of Microsoft Office.