Convert Stand Alone Windows Application to Subscription - Authentication via website

I currently have a completed windows software program in both build and source form. The program is distributed to users by way of an installer program and then unlocked using a key created by a keygen. I want to change it from a standalone product to a subscription service. I will be selling the software as a subscription based model. What I want to do, is modify the program so that a user needs to log in using a username and password in order to use the program. The program will be left entirely untouched, except you would be adding an authentication process to use the program. The program will no longer need to be locked or unlocked with a keygen, but instead will require a user to sign in using their username and password. User account would be stored on my server. I would need a simple backend that allows me to turn on and off user account, both in the present, or after a predetermined time, and generate reports of user activity. The system will prevent multiple sessions from being initiated by the same user at the same time. I would not be against a 3rd party system such as bluesnap, though want to maintain the ability to directly control user accounts in my system. I would want a rather quick turn around on this, since the project doesn't really include modifying the existing program, only putting a wrapper on it for subscription service. Please let me know how this sounds. Regards, Brendan