Mar 29, 2019 · Add the new pack key to Windows 8.1. Once you have the key, you can enter it in Windows so that your upgrade can be downloaded and installed. Press ⊞ Win and type "add features". Select "Add features to Windows 8.1". Choose "I already have a product key". Enter the key into the field.. Windows Defender is free, as it comes built-into Windows 8.1. However, if you’re looking for more comprehensive security software, Avast offers a free antivirus for Windows 8.1. The free version of Avast Antivirus for Windows 8.1 provides powerful and comprehensive protection that should more than suffice for your needs..