I purchased Adobe acrobat 8 std about 3 years ago. I attempted to install the product with the serial no. indicated on the package. I recieved a message during my attempted installation "serial no. not valid". I also have a 24 digit activation no. He

I purchased Adobe acrobat 8 std about 3 years ago. I attempted to install the product with the serial no. indicated on the package. I received a message during my attempted installation "serial no. not valid". I also have the 24 digit activation number. Can you help me with the installation?