I've been using older version of Avast (~1 year old). I had only antivirus module installed and it had disabled Windows Defender automatically. Now i have updated to the latest Avast version and Windows Defender is regularly showing me definitions updates to install. Usually antivirus programs take over that part of Windows security. Wonder why this time it didn't. Granted i have tried out Microsoft Security Essentials before installing the newest Avast version, so maybe it changed something on a system. But i have done this also a year ago while testing various antivirus products and WD wasn't active after Avast install.