You can allow Sync through Avast Antivirus.
Here's how:
1. Go to 'Menu' then click 'Settings'
2. Select 'Components'
3. Click on 'Customize' for Behavior Shield
4. Select 'Exclusions' 5. Click on 'Browse'
6. Navigate to: C:\Users\yourusername\AppData\Local\Programs\Sync\ Note: Replace yourusername with your actual Windows Username Select both 'sync-taskbar.exe' and 'sync-worker.exe'.
7. Click on 'OK' and close Avast