Buy
Microsoft Office Excel 2007
Excel 2007 & Excel 2003 Dual Installation Notes
It is
Highly Recommended that, if you are considering upgrading to Excel 2007 from a previous version of Excel, you keep the old version of Excel and install the new version along side of it. This can be done with all versions of Excel; however, not all installations have gone smoothly.
Recommendations: 
Install Excel 2007 while keeping all previous versions of Excel.
Make sure that your Excel versions have been or are being installed earliest version
to latest version so that Excel 2007 is the latest edition installed.
Install the different versions of Excel in different directories on your computer.
Always Backup all Excel and Microsoft Office files before updating or installing other
versions of the software.
Always make sure to create a restore point before installing any new software.
Benefits: The main benefit of doing this is that you will ensure full compatibility with any and all
excel files that someone might send you.
If you are not as comfortable using the Excel 2007 Ribbon menus, you can just use
Excel 2003 or earlier.
Ensures the highest level of usability and compatibility.
