How to install Avast Free Antivirus on Windows | Avast 02.06.2022 · If prompted for permission by the User Account Control dialog, click Yes.; To change the default setup language, click the current language in the top-right corner of the screen. Then, click Install to proceed with default installation, or click Customize if you need to make changes to the default setup.; Wait while setup installs Avast Free Antivirus on your PC.