how do I check the icon cache?
I don't remember the exact method on windows Seven, but essentially you need to check the properties of the task bar -> notification area -> customize -> Avast -> show (behaviour).
Alternatively , there are a couple of registry keys, but I need to double check them (so probably I won't post them, but this might change according to your feedback and my own time). There are alternative GUI tools to do the same.
Start with the above "instructions" to see if the tray icon is hidding "itself" (well, Windows does it). Or "show" (unhidden) all tray icons in the notification area properties.
All this may be useful only if:
_ the display drivers are working correctly and updated; and
_ the screen resolution (and number of colors) is "supported" by the avast tray icon; and
_ other tools are not conflicting with avast (whether it is DSBridge, sound drivers with tray icons, remnants of other antivirus tools, theme tools or "docking" tools, or...).
Just to mention a strange example, there was a case in this forum where a certain Windows font was causing GUI problems in avast. Once the specific font was deleted, avast GUI return to normal. So there are many things to double check for conflicts.