cannot turn on firewall

[dragonfly1958]

Having installed the new version of Avast (20 day trial) I have a message for my attention saying my firewall is not switched on.
However, whatever attempts I make to turn it on just ptompts the message 'unable to start firewall'.
Any suggestions gratefully received.

[Asyn]

Repair avast!:
1. Control Panel -> Add/Remove programs -> avast!
2. Click on 'Repair'.
3. Follow instructions.
4. Reboot.