If you are a user of any of the paid versions of Avira's various antivirus and security software and you have tried to update it/them in the last 24 hours, chances are that you're now sitting before a crippled PC, wondering what happened.

So what did happen?

Well, it seems that the new update makes the AntiVirProActiv component - not present only in the company's free offering - erroneously detect critical Windows processes as malware and automatically terminate them.

It also blocks other popular Microsoft and third party software, and sometimes even prevents Windows from booting at all.

It is unknown how many individuals and businesses were affected by the defective update but, according to Emil Protalinski, it seems to have been downloaded millions of times.

Avira's forums are heaving with users searching for a way to undo the damage, and the company is furiously working on a solution.

So far, they advise users to either temporarily disable the ProActiv component or to add an exception for every blocked application.

For those who can boot Windows only in safe mode, disabling ProActiv requires bringing up the Task Manager, opening a new task and typing “c:\program files\avira\antivir desktop\avconfig.exe”, then running it.

This opens the Avira Antivirus configuration window where, in Expert Mode, ProActiv can be disabled by unchecking the "Enable ProActiv" option, applying the change, and restarting the computer for the change to take effect.

These inelegant solutions are expected to be dropped once a new update fixing the problem is issued.