firewall won't turn on

HELP! My firewall is turned off and it will not turn on. When I click the link to turn it on, I get a nasty message that says, “Unable to start Firewall!” I restarted my computer, but I keep getting that message. What can I do to turn it on? Thank you!

do you have any other security programs installed…like another AV?
did you remove your previous used AV before installing avast, and used that vendors removal tool to clear any leftover files?

have you tried avast repair?
controlpanel > ad/remove programs > avast > uninstall/change > repair option … wait a minute and reboot

any change?