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,

  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
    See picture here: http://forum.avast.com/index.php?topic=26155.msg213891#msg213891

If that does not help, please, uninstall, boot, install again, boot.