In order to ensure that the registration is unique, you'd probably need to set up a server that would validate the registration information (that Megatron indicated) _or_ (and this will be a lot harder) you'd have to mkae the software poll for other installations of the registration number (something like what tcp/ip does when it finds a duplicate ip address). Or you can do what most game manufacturers do, require a CD (or even a dongle).

In any event, you will only be able to slow down piracy, not prevent it. No matter what you do, if your software is popular enough, your protection scheme will be cracked. Copy protection only protects you from casual theft...