newer versions open files created with older versions. So it's possible these very old files are corrupted somehow. But it's possible also, indeed, that these files are fine, but incompatible indeed.

Quattro Pro has never been free, so I doubt if you can find downloads of old versions. Things to try:
- see if Excel (with all conversion modules installed) will open it. You might try to install Office 95 or Office 97 for the purpose.
- see if OpenOffice Calc will do it
- contact Corel. The helpdesk might be willing to accept your files and convert them.