I am working on a secure web site, that will allow the user to register their computer, to avoid answering the primary security questions.
Is this just placing a cookie on their machine, and checking to see if the cookie exists?
Or do I need to retrieve the client machines ID data?
You may use a
combinationofcookies,encryptionandhashing algorithmsat your site to implement this.Having said that, you should not save critical information bits on client computers and just save basic authorization and authentication details.
For example: Most of the websites provide you with the facility to remain logged in for specified period like yahoo mail etc.
you can use Tickets as well. See the following tutorial for details
Forms Authentication Configuration and Advanced Topics