Avast internet security 2015 - cannot turn firewall on

I had been running Avast free software (which always worked fine for the last year). Prior to this I had used AVG, which I removed using ‘remove program’ in windows Control Panel. I’m running windows 7 Home. I decided to upgrade to paid version to get the extra coverage. However, having registered and downloaded the licence I repeatedly get a message that says ‘Attention, Firewall switched off’. When I click ‘Turn On’ I get a pop up that that simply says ‘Unable to start Firewall’.

I’ve spent too long (3hrs) trying to resolve this and without a firewall I’m feeling very vulnerable! I almost feel I’ll be better to write the £25 cost off and install windows own AV or go back to AVG.

There must be a fix! HELP!

Removing through the control panel is not enough.
http://www.ache.nl/index.php?location=mal-01
Use the removal tool also.

Remove avast also completely and perform a clean installation of the latest version.