(in Windows XP)
Right click the bottom taskbar, click "properties" make sure the box "hide inactive icons" is NOT ticked, click OK.
Does the icon appear?
Open Taskmanager (ctrl-alt-del) and check that the following processes are there: ashDisp.exe, and ashServ.exe. There may (probably) be more "ash" processes, depending on how many providers are running, and how you've configured it.
If not, go to "add/remove programs" in the control panel, click on Avast, click "change or remove",select "repair" at the options.
 posted while "oldman" was posting his reply. Check his suggestions first.