BitDefender's turn this time round:

http://www.securitynewsportal.com/cg...0out%20Windows

Apparently the problem was caused by a dodgy virus definition which falsely identified a number of essential system components as a well-known trojan, causing the OS to crash and fail to reboot.
Embarrassingly, it seems that the virus killer update also identified components of its own software as harmful, effectively making it commit suicide.
It appears to only affect 64bit versions of Windows.