You have said Files, (plural), rather than File.
The files may be corrupt,but if it is several files that seems unlikely.
Is it ONLY Excel files that exhibit this problem? If so I'd venture to suggest that perhaps they were created in an later version of Excel. E.G files created in Excel 2000 wont open in Excel 5... Could that be possible?
Do the files open, from that CDRW, in another machine? If so it suggests it's not a problem with the files on that disc.
Can you open other files on the same disc?