Does your old registration number still exist?

If you're looking for your Windows registration code, there are programs which can find it for you if you've lost the card (one that can be downloaded for free goes by the name of "magical jelly bean keyfinder" and I wish I was making that up but I am not). Note that you're not allowed to use it on a different computer, but it is permissible to upgrade to a larger hard drive or reinstall it on the same computer. You may need to call Microsoft to "activate" it.
If that's not what you meant, please be more specific about what you mean by "registration number."