Old Photoshop files

I have CD art on old Zip drives. They have been copied to a PC if I recall and transferred to a Mac. I am sure they opened years ago, but will not open on mac or pc now . The files were created around 2000. 

They say they are psd files, look normal and then something like not valid photoshop files. Any ideas?  I have an old photoshop CS3.  The one used was probably older? 

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