You got an Excel 2007 spreadsheet in your Inbox but there’s no Microsoft Office on you computer to open that file.
You could either buy Office 2007 or just grab the free Microsoft Excel 2007 viewer that was released this week. This program has been available since long but only the new version supports the xlsx format.
With this new Excel 2007 Viewer, you can view and even print spreadsheets created using any version of Microsoft Excel. You can also copy data from Excel Viewer to another program.
The other option for opening xlsx files is the Office Compatibility pack but that works only if you have a copy of Office 2003 or Office XP.
Excel Viewer [Microsoft Download Center]
Related: Convert XLSX to XLS [using Zamzar]
