Pattigayle, are you running any spyware monitor (TeaTimer from SpyBot, Ad-Watch from Ad-aware, PrevX, etc.) while installing avast?
You could be avoiding the startup registry entry for ashDisp.exe...
If you run it manually from avast folder, will the icon appear?
You could stay with AntiVir but never with the resident shield enabled (I think it's called Guard in Antivir)...