Ron, I have Excel 2003 and 2007 on this computer. I did a test where I saved a file in 2007 and then opened it in 2003. Previously, I had downloaded the compatibility pack from here. It runs through a conversion mode, but you can edit the file. If one of the new features was used, you get an error message, but it looks like you can still use the spreadsheet once you deal with the error.

The link Hayburner gave is if you don't have any version of Excel. You can download a Reader program that allows you to view and print only.

MS added a lot of functionality to the new version of Excel. Way more rows, new functions, different formating options. They also seemed to want this file type to be "open". It looks like their 10 year old format finally ran into serious limits.