I have also noticed this a few times lately (last time today).
I (a) icon disappears, and when I try to start “ashserv.exe” (presuming that avast! has stopped) I get an avast! ball icon with a small red dot. If I try to enter the avast! setup from the icon I get “wrong password” even if it’s right!

I run Win98se. Problem disappears if I boot, but reoccurs from time to time - and an anti-virus that suddenly stops…

Update: Strange. Not long after the avast! icon disappeared ZoneAlarm shut down as well (red icon with yellow X) and when I open ZA it says “system error, please reboot”. What is happening?