1. Check the option in the Appearance tab of settings.
or
2. Repair your avast installation through Control Panel.
or
3. Make a link to ashdisp.exe in your startup folder
or
4. Add the path to ashDisp.exe into a value named avast! in the Windows Registry key HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Run
Are you by any chance using spysweeper?
If so, use the restore/recover or whatever it may be called to restore the deleted registry entry and update your definitions file for spysweeper. It reports incorrectly (false positive) the avast ashDisp.exe which is the icon you see on your system tray.
Other programs that can block are the startup monitors: PrevX, ProcessGuard, WinPatrol, TeaTimer, Ad-watch, etc.