30 Demo Timer

Hello,

Does anybody know of a class that will allow me to create a secure 30 day time limit for a demo application I am creating? If there is no class, what is the most accepted way of allowing the user to use the product for a limited time, and then (when the time is up) disallowing the user from using it, or re-installing it and using it? Do people typically modify the registry?

Thanks a bunch