BitDefender apologizes for a faulty update.

BitDefender launched yesterday a security update for all platforms. The problem is that it has an unnormal behavior under x64 version of Windows. It detects multiple files from Windows and from BitDefender as being infected with Trojan.FakeAlert.5, so they are quarantined. And this is what happens: many of these computers are unable to boot anymore, or if they are able to boot, other vital applications have higher chances to fail.

This update has been removed from download servers, so if don't have any problems, you can update BitDefender. If you are experiencing issues and eventually you can't boot, instructions can be found by clicking here. You may also try a system restore if you can't boot (if possible).