I have lots of old Excel workbooks that were created on my Mac that go back as far as 1999 at least. There were created in 1998 Excel 8.0 (from Office 98), 2000 Excel 9.0 (from Office 2001), and 2001 Excel 10.0 (from Office v.X) (I'm assuming these were the versions I used way back then because I have always used Office since the beginning of time.) Excel 2011 will not open these files. When I try to open these files, I get the message:

Excel could not open [filename] because some content is unreadable. Do you want to open and repair this workbook? If excel cannot repair the file, some data may be lost.

If I click Open and Repair, Excel hangs.

Of course, being an ancient Mac user, these files do not have those ugly extensions hanging on the file name, but that really isn't the problem. I can add extensions to the file name, which changes the appearance of the icon, but the Excel still hangs on trying to Open and Repair these files.