I have recently encountered three PCs (one Windows Vista and two Windows 7) which have each suffered a failure of Avast. With the two Windows 7 systems (both 64 bit) the Avast icon in the system tray had a red marker and it was found that the Avast service was not running. The Fix option in the Avast GUI had no effect. It was necessary to uninstall, clean and reinstall Avast and all is now well. A similar thing happened with Vista (32 bit this time). The user mentioned to me that the Avast system tray icon was missing. This time however, Avast responded to the Fix option and restarted.
I mention this in case others have noticed similar behaviour. I run Windows XP Pro and Avast has not yet misbehaved in this way for me.