Left clicking the avast a icon (double or single depending on your OS settings) will only bring up the On-Access Scanners/providers interface. You can only access the Start avast! Antivirus via the right click menu. The reason for this I assume is that this also generates the memory scan, no point doing a virus scan if the memory is infected.
Why is it that you are trying to get quick access to the on-demand scan/simple user interface ?
You could do as I have done and create a desktop shortcut to "C:\Program Files\Alwil Software\Avast4\ashSimpl.exe," right click on that file, Send To, Desktop (create shortcut). This doesn't do the memory scan prior to display so is unsuitable if you want to do an on-demand scan. I use it more as a quick way of getting to the Menu in that Simple User Interface without having to go through a memory scan (or disable that).
