We suppose one way to protect users from a malware infestation is to prevent Windows from loading in the first place, and that's exactly what BitDefender did, though not on purpose. Borked WIndows installations were the inadvertent result of an overzealous definitions update that mistakenly detected several windows and BitDefender files as infected with Trojan.FakeAlert.5.
"We apologize for the issues that you are experiencing because of an update released today for Windows 64-bit systems," BitDefender told its customers. "The faulty update has been removed and we are quickly working on a fix for the issue experienced by the users that downloaded the update."
BitDefender has since issued a software patch along with instructions on how to apply it if your system refuses to boot. The company also warns that "there have been several 'self help' articles on the Internet that do not fully solve the problem" and is instructing users to only follow BD's official instructions.
More info here .
We talked to BitDefender about this issue and were told that "due to the fast reaction with a reversed update after the faulty one, there has only been a few hundred machiens that were critically affected; however at this moment many of them have had one-to-one support."