Ever since installing the 4.8.1049 program update for Avast Server Edition this morning, Exchange Server 2003 has been shutting down on its own. Thereafter, the Avast Exchange 2000/2003 provider shows that it is "waiting for a subsystem to start."
Rebooting the server corrects the problem for a while, but Exchange Server eventually shuts down again, one or two hours after the reboot.
Before the 4.8.1049 program update this morning, I have never experienced a similar problem with Avast or Exchange Server.