Do (or did) you have another Antivirus installed on this system, if so what (and how did you get rid of it) ?
What additional security based software do you have, e.g. AdAware, Spybot, winpatrol, prevX, etc. anything that would either monitor or block new start-up entries ?
As a temporary measure you can create a desktop shortcut for C:\Program Files\Alwil Software\Avast4\ashDisp.exe until we resolve the problem. That when you double click it will give you an icon.