Give this a go:
It will only work if you have Avast installed in the default directory:
Open Notepad, copy and paste the following text (in bold) into the new Notepad window.
Save it to your Desktop,
save as file type
all files
save as
avastrun.bat
Code:
@echo off
cd C:\Program Files\Alwil Software\Avast4
ashAvast.exe
CLS
Exit
Now double click this file on your desktop.
This should start the avast GUI