Solution

If you're using Windows XP, the icons are probably hidden. In that case right-click on your Taskbar and choose Properties. A dialog box "Taskbar and Start menu properties" would appear. Then uncheck Hide inactive icons check box. If you´re not using Windows XP, OR if the previous solution doesn't fix the problem, run the ashDisp.exe program (it´s in C:\PROGRAM FILES\ALWIL SOFTWARE\AVAST4 directory).
It should bring the icons up again. If they disappear after restart again, make a shortcut to the ashDisp.exe file in your Start-up folder.