1. Deactivate it on the old computer (Help > Deactivate…)
2. Install it from the original installer with all updates again. You will need the Serial Number, either during installation process or when the progamm launches the first time.
If you have no possibility to deactivate it again you should call Adobe Technical Support. They will help you.
Check your serial number on your Adobe account (if you ever bothered to register your software, that is). If you don't have any, then I'm afraid there is nothing anyone can do. Otherwise follow the procedures Willi explained already.