If you set up too small timeout for virus scanning in SharePoint Central Administration (Security Configuration > Configure antivirus settings), there may occur situation when SharePoint shows "The installed virus scanner is currently unavailable. If the problem persists, contact your administrator" message every time you try to download/upload some document.
This issue does not occur inside AVG as AVG only relies on SharePoint
programming interface. The solution is to increase timeout for
documents scanning (we recommend the default value - 300s) and restart
the World Wide Web Publishing Service.