A method is described for controlling customer installations of software or data by providing to the customer an encrypted list of authorized installation targets, whereby the installation program reads and decrypts the list, and only allows installation to proceed if the customer's installation target has a serial number that matches one of the vendor-provided serial numbers in the authorization list. Provision is also made for allowing customers to add serial numbers to the list, within constraints predetermined by the software vendor. Also provided is a method for a customer to perform a predetermined number of installations, whereby the software maintains and decrements a counter in an encrypted file on a storage medium, keeping track of how many remaining installations a customer may perform.