In the End User Licensing Agreement (EULA) it is stated that the maximum installation number is 3, which means that you can install the application on 3 devices at a time. But if you completely uninstall it on one of them the count decreases to 2. If you know that you have installed the application just once, then uninstall it and download another version of Home & Student on Windows 7 and activate it using the same serial key.