Application Security

I wrote an application that I require people to purchase and I was wondering what the best approach would be to limiting the use of my software to only those who are allowed to use it.

I was thinking about using registry keys but in the end it's not such a good idea seeing as most anti-virus programs will pick up on registry modifications which would cause the buyer to be unsure about my software.

I've also thought about using a database to where I would manually add PayPal e-mails and would have the buyer input their PayPal e-mail address in my software before they could use it. Problem is this isn't an instant process and would like my customers to be able to use my software immediately after the purchase.

I'm not asking for a step-by-step process on securing my application I would just like a step in the right direction.

Thank you!

You could probably find a lot of the stuff searching the forum for something such as "Application Security" useful! This question tends to be asked and answered a lot so there should be quite a few ideas for you to try.