This is something I have been wondering for a while. If you have a trial program installed and your trial expires, then you un-install the program and re-install the same trial, it knows that it has been on you computer before and prompts you for a license key. How does it know even if all traces of the program have been removed from your computer? My best guess is when a trial is installed some information unique to your machine is sent to the publisher, when you try to install it again it checks this database for your machines ID to see if you have already had it. Also please don’t misinterpret my post, I am by no means looking for a way to bypass this, it’s just strictly curiosity.