Had my first run-in with .xlsx this morning. One of my customers sent me a file saved with Excel 2007 and my Excel 2003 couldn't open it. I found a Microsoft download that solves the problem. Microsoft Office Compatibility Pack for Word, Excel, and PowerPoint 2007 File Formats.Knowledge Base article 924074 ''By installing the Compatibility Pack in addition to Microsoft Office 2000, Office XP, or Office 2003, you will be able open, edit, and save files using the file formats new to Word, Excel, and PowerPoint 2007''