When I try to register mi photoshop account, there's a problem with the internet connection and I can't continue. How can I solve it?

I bought a book about Photoshop CS many years ago, and once I've installed it in my computer it says I should register the Photoshop account in order to use it (or something similar), and it also has a link to go directly to the register site. The problem comes when I try to access to that link given, which is an internet link. It appears in the screen that my Internet connection is not secure, so I'm not allowed to continue with the process, even though I really don't have any problem with the connection. As I'm talking about an old Photoshop CD version, the problem might be something related to incompatibility with my microsoft version. How can I solve this problem, if there's any solution possible?