Sometimes there are compatability issues with older versions of programs when we try to open them with newer versions. I would suggest going to the microsoft site and checking whether this is the case. If it is they would have developed a patch that would overcome the problem you are having.
I've had the same problem. Once I start using software I hate to upgrade computers or operating systems because of this. So if it ain't broke, I don't fix it. If can can find someone with an intermediate version of Excel you may be able to step the files up to that version and then step them up to your current version.
I moved to the new version of Excel but my files are .xls. Until now I had no problem to open them and to convert them into the new excel format. By the way, I didn't hear anyone having problems with it.
I would suggest going to microsoft's website and looking for updates or installations that could deal with compatibility issues. Or if really needed search google or download.com for a software that allows to open older files of Excel. Good Luck!
When I switched from Office 2003 to Office 2007 I had to download a file converter from the Microsoft website that converted the 2003 version into readable files for version 2007. One would have to believe that such a converter exists for the excel program as well?