Just to add that I've today been bitten by this bug. I was running Avast Safezone for the first time, and I was unlucky enough to be doing this on a dual monitor setup. Sure enough, the minimise and close buttons were glaring in their absence. I tried a few things to close Safezone, including running a command prompt with taskshow and taskkill, ctrl-shift-escape and ctrl-alt-delete to get at task manager. Nothing worked and eventually I was hit with a blue screen. "Oh well, that sorts that", I thought, and followed through with the reboot. On boot I was presented with another blue screen, "BAD_SYSTEM_CONFIG_INFO". Another reboot and the same blue screen again... and agan... and again.
For anyone who faces this, you'll probably be fine with a "Restore Last Known Good Configuration" or, if like me you're on Windows 8, an automatic repair which is presented to you after a failed boot. Unfortunately, I have an encrypted system drive (Truecrypt). So I first had to backup the contents of the drive by moving it to a different PC (for safety), and then decrypt the drive so I could run the Windows 8 recovery tools.
12 hours later and I'm back up and running.
Pretty frustrated... and sufficed to say that I've completely lost confidence in the Avast Safezone. I think the Avast team need to look at this bug as a matter of urgency!