If you can't uninstall the Behaviour Shield this way you will need to do a clean reinstall below.
From the Windows add remove programs, select Avast xxx, click the Change Remove button; from the next window select Change and deselect the Behaviour Shield, OK your way out and reboot.
If you can't just uninstall the behaviour shield:
Do a clean reinstall, but elect to do a 'custom' install, this will allow you to select what components/shields, deselect the Behaviour Shield and complete the installation.
This is the problem area, conflicting with the wlan card, so by not installing it as a temporary workaround, should resolve the problem.
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They are working on a fix and a program update should be released soon (I don't know), once it comes out you can reverse that process and install the Behaviour Shield. From the Windows add remove programs, select Avast xxx, click the Change Remove button; from the next window select Change and check the Behaviour Shield again, OK your way out and reboot.