You maybe best to contact Microsoft and ask if a product key can be supplied. Your disk should have a serial number, they can use that to identify authenticity with some disks. They may charge a fee for this or due to it being old software, they may not be able to help. Without the product key code there really isn't much you can do. You can't upgrade or install a version of Office without the product key code.
Your only other alternative is to install Open Office for PPC Mac.
Open Office will open and edit Microsoft Office files.
Select the correct install for your version of Mac OS X.
Bets of luck