I'm using Avast Internet Security and this worked for me. The problem is not with the Avast software. When Windows 10 is installed it selects Windows Defender as the default antivirus/Internet security program. This blocks the Avast software from functioning properly. To fix this, you will need to go into the Windows 10 settings pages, and completely shut down Windows Defender. Open up the settings page and look for the securities tab. Open up the securities tab and look for Windows Defender in the menus sidebar. There are three items on the Windows Defender homepage: Real – time protection, cloud – based protection, and Sample submission. Turn off all three, close settings, and restart your computer. This should allow your Avast software to start up properly and block Windows defender.