what we do in one of our application is to store the mac address of the system. once the system open it will show mac address and ask for license key, now user will call the support team and ask for key , we use some encryption/decryption mechanism to generate a key based on the mac address. we provide this key to user, once he enter the key and submit this entry will save in their database. so next time when the user login system will find that mac address and the key based on that will be saved in the database so it will not ask for license.
you can read below link on how to get mac address
get MAC address in message box using c#[
^]
Submit an article or tip