Error: "Excel can not open file XXX because the file format or file extension is not valid. Verify that the file has not been corrupted and that the file extension matches the format of the file"

I updated my office 8 to office 10.  Now I can't get into any of my files, I think due to conversion.  I must have done something.  I uninstalled office 10 and reinstalled 8.  I still can't open my files.  The error message I get is "Excel can not open file XXX because the file format or file extension is not valid.  Verify that the file has not been corrupted and that the file extension matches the format of the file" .  How do I do this? 

Original title: Corrupted files