German security firm Avira yesterday issued a service pack for its antivirus software that crippled an unknown number of Windows machines, with one customer calling the gaffe "catastrophic" to his company.

Today, Avira updated the software to sidestep the problem.

"Following the release of Service Pack 0 (SP0) for Avira Version 2012, the ProActiv feature blocked legitimate Windows applications on customers' PCs," Avira acknowledged on its support site. "We deeply regret any difficulties this has caused you."

Avira is the world's second-biggest antivirus maker, according to usage statistics.

The service pack included an update to ProActiv, a behavioral-based monitoring system that watches for suspicious events that may hint at a malware attack or point to an infection.

Users quickly reported that the updated ProActiv was blocking almost every legitimate Windows executable file -- those with the ".exe" extension -- meaning that most applications refused to launch. Even worse, ProActiv prevented critical Windows files from running, which in many cases "bricked" PCs, or kept them from even properly booting.

The inadvertent blocking impacted Avira Professional Security, Avira Internet Security 2012 and Avira Antivirus Premium 2012, paid products priced between $30 and $60. Avira's free antivirus software, which has limited functionality -- and does not include ProActiv -- was not affected.