Running Avast 6.0.8 r130520 on Windows 7 Professional 64-bit. I use VirtualBox to run VMs, and it has been working fine for years. Then just today, trying to start my largest and most important VM would crash every time. I ran it 2 days ago without issue. I did some searching about "virtualbox crash", and found a link about disabling "Enable hardware-assisted virtualization" in Avast. I did this and the problem went away. The VM runs Ubuntu 64-bit. I have a 64-bit AMD CPU (Opteron 4234) with hardware virtualization enabled.
I can only guess that Avast updated itself and turned on this hardware-assisted virtualization feature. Please default this setting to OFF for existing installations.