You can go to steam, purchase a new copy for a new account. Log into that account in windows and you can right click on Arma2 to copy the key, then use Darkly's to change it.
Or you can start about a 2 week process to get a new key for free assuming your key got stolen and you were not the one hacking with it. Start by emailing Battle eye and eventually they will direct you to Bohemia who will need proof of purchase of Arma II (a copy of your steam account with purchase date and price is sufficient) and they will email you a new steam activation code which can be used to activate a new copy of ArmaII - Legit, unhacked key.