Try disabling "Enable hardware-assisted virtualization". That solved it for me.

I manually updated Avast Free when I read about the Project Zero security hole in Avast. I then rebooted and got a BSOD in Win7 SP1 at boot. Over and over. The error code of 124 is usually a flaky overclock. So I tinkered in the BIOS.
Finally found that disabling virtualization in the BIOS fixed the BSOD, but then I couldn't use my VM's, so that's no good. Found the virtualization option in Avast, disabled that, re-enabled virtualization in the BIOS, and presto. No more crashes. The overclock was and is fine. Avast is just broken again.

I think it's weird this option is even enabled by default, since I don't have any of the functionality installed that uses it, such as the built-in sandbox.