Prevention of Sharing USER ID nd Password for a paid website

Hi there, We are shortly launching our website for performing calculations online. The website subscription will most probably be made on paid basis after a certain period upon launch. One of our requirements is to prevent the sharing of user D and password by the subscribers. For this purpose, we want the subscriber to use the same computer that he/she uses for subscription purposes. So if one user wants to use both his personal laptop and his work station, he/she needs to get two separate subscriptions. For this purpose, I suppose we would need a small programme to be installed on user’s computer while the subscription form submission is in progress. I would like to hear any suggestions from you on this mechanism and would like to know as to whether you will be able to provide any suitable solution for this requirement that can be incorporated in our programming code. We look forward to getting the solution to the above.