I had this problem when I updated from V8 to V9. I have Windows 7 professional 32-bit SP1

What I did is:

1- Restart in safe mode then change in the registry “C:\Program Files\AVAST Software\Avast\AvastUI.exe” /nogui to be: "C:\Program Files\AVAST Software\Avast\AvastUI.exe /nogui"

2- Restart normally. Make sure that Avast is not started (check services in Task manager)

3- Restart in safe mode then change in the registry “C:\Program Files\AVAST Software\Avast\AvastUI.exe /nogui” to be: “C:\Program Files\AVAST Software\Avast\AvastUI.exe” /nogui

4- Restart normally then everything was working fine with Avast.