German security company Avira is experiencing serious technical difficulties.
A defective antivirus update that has been downloaded millions of times is bringing Windows XP, Windows Vista, and Windows 7 computers to a screeching halt across the world, according to user reports (1, 2).
Avira has released an update that reportedly fixes the issue, but users are still having problems.
The moderator says the update fixed the issue for him, but not everyone in the threads agrees.
I would recommend trying to get into Safe Mode, disabling Pro Activ, rebooting Windows, updating the antivirus, and re-enabling Pro Activ.