Serial / Activation Key Protection

[BowHunter]

I need something that will prevent users from sharing my program. I have looked at several different ones, but they are a bit expensive. Does anyone have any suggestions? I would lik the ability to automatically provide a serial / activation key to my customers once they pay and I receive the IPN from Paypal. I want to tie the activation key to the user's computer to prevent them from sharing the program with their buddies. What do you folks use to accomplish this?

Thanks,
BowHunter