* Do you have another version of Orcad? Try to install it and you may be successful. Then try to upgrade to your newer version.
* The registry is still suspect. Somewhere it may contain one or more entries which are Orcad related. When the installer spots these entries, then it throws up the error message. If you do a lot of searching you may find step-by-step instructions to edit the registry, so as to remove the last evidence that Orcad is installed.
Unless you have detailed instructions it's a bad idea to touch the registry. If the registry becomes corrupted, the computer may cease to operate properly. Then you would need to do a fresh Windows install.
* This is a possibility. There may be some file which the installer creates on your hard disk (maybe in your system folder). The application looks for this file each time it runs. The file may contain information about your license, valid start and end dates, etc. If you can delete this file then that may permit you to install Orcad again.