Edit /etc/rc.local adding the following before the "echo '.'" line, you
should also call freshclam.

if [ -x /usr/local/sbin/clamd ]; then
	echo -n ' clamd'
	# Don't allow files larger than 20M to be created, to limit DoS
	# Needs to be large enough to extract the signature files
	(ulimit -f 20000 && /usr/local/sbin/clamd)
fi
