.xslx,.pptx,.docx files are excel,power point 2007 files.If you want to open these files in office 2003 and other MS office versions you just need to install Microsoft Office Compatibility Pack for Word, Excel, and PowerPoint 2007 File Formats.
Users of the Microsoft Office XP and 2003 programs Word, Excel, or PowerPoint—please install all High-Priority updates from Microsoft Update before downloading the Compatibility Pack.
By installing the Compatibility Pack in addition to Microsoft Office 2000, Office XP, or Office 2003, you will be able to open, edit, and save files using the file formats new to Word, Excel, and PowerPoint 2007. The Compatibility Pack can also be used in conjunction with the Microsoft Office Word Viewer 2003, Excel Viewer 2003, and PowerPoint Viewer 2003 to view files saved in these new formats.
Download MS Office compatibility pack from here