avast! icon missing - As a temporary measure until this is resolved you can create a desktop shortcut for this file C:\Program Files\Alwil Software\Avast4\ashDisp.exe (the avast icon and interface to the providers). Right click on the file and select Send To, Desktop (create shortcut). You will need to run this after each boot until the problem is resolved.
What other security based software do you have that might block new startup entries, e.g. Spybot S&D (TeaTimer), AdAware (AdWatch), SpySweeper, Spyware Doctor (StartUpGuard or OnGuard), PrevX, WinPatrol, ProcessGuard, etc. ?
Check the option in the Appearance tab of Program Settings. Or Make a link to ashdisp.exe in your startup folder.
Have (or did) you another AV installed in this system, if so what was it and how did you get rid of it ?
If none of the above are applicable, then a clean reinstall would probably be best, as a step back with system restore can produce unexpected results.
Download the latest version of avast (if you didn't save it before)
http://www.avast.com/eng/download-avast-home.html and save it to your HDD, somewhere you can find it again. Use that when you reinstall.
Download the avast! Uninstall Utility,
find it here and save it to your HDD.
1. Now uninstall (using add remove programs, if you can't do that start from the next step), reboot.
2. run the avast! Uninstall Utility, reboot.
If step 1 failed it may be necessary to run this from safe mode, once complete reboot into normal mode.
3. install the latest version, reboot.