If you don’t have a product key, the best legal option is to purchase a genuine license from Microsoft or an authorized reseller. Microsoft offers different licensing options for Windows, including digital downloads and physical copies. Additionally, you might consider exploring Microsoft’s subscription-based service, such as Microsoft 365, which includes both software and updates. If you are a student or work in an educational institution, you might be eligible for discounts or free licenses through educational programs. Always ensure that you acquire software from legitimate sources to avoid potential issues and to receive proper support.