Put your install disc in your drive and install
That would work...however, I downloaded the software and did not register it.
Just download it again and reinstall.
Welcome to the forum.
Do you still have the downloaded files?
Did you buy from the Adobe Store? If so, they should have a record, and also the link for download, if you did not backup those files. Remember, there are two - the program, and the Additional Functional Content, which can ONLY be installed after you have installed the program.
When you download software, you should ALWAYS write the download file(s) and a copy of your serial number in a text file to a CD