The grey is a blue grey, but never mind that, check that the Program Settings, Appearance, Show avast! tray icon is ticked. Ensure that it isn't hidden (Hide inactive icons).
If it is none of those have you installeed any security based programs recently ?
What other security based software do you have that might block new startup entries, e.g. Spybot S&D (TeaTimer), AdAware (AdWatch), SpySweeper, PrevX, WinPatrol, ProcessGuard, etc. ?
avast! icon missing - As a temporary measure 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).