Do you mean that the icon is there but does not work?
Or do you mean you don’t have the icon?
If is the second,
- Check the option in the Appearance tab of settings.
or
- Repair your avast installation through Control Panel.
or
- Make a link to ashdisp.exe in your startup folder
or
- Add the path to ashDisp.exe into a value named avast! in the Windows Registry key HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Run
See picture here: http://forum.avast.com/index.php?topic=26155.msg213891#msg213891
If that does not help, please, uninstall, boot, install again, boot.