Bitdefender had issued a bad update last night which is named as "Fakealert.5" now. What happens is that it basically start tagging most EXE and DLL as trojans and as a result starts to quarantine the majority of your files from MSN, Skype, Windows components to your Antivirus itself. Now this is bad.
With the files being qurantine, it gets moved and locked away causing the system to crash upon reboot.
Bitdefender site has more information on this issue:
Bullguard, which also happen to use the BD engine, had also more information here:
I suggest that if you are using any of these, please do not reboot yet. Read the above carefully and make the necessary fixes before rebooting in order to save your PC.