Your only opportunity to fill in and submit the Publisher online registration form is when you start Publisher for the first time. You can not register online again, even if you uninstall Publisher and then reinstall it.
If you choose not to register when you are prompted, the only way to register is to fill out the registration card that came with your copy of Publisher or Office 2000, and send it by postal mail to Microsoft.
Once you register, Microsoft will notify you by mail or e-mail about product updates, new Microsoft products, and other Microsoft products that may interest you based on the information you submitted during registration.