InfoWorld reports that a flawed signature update to AVG Technologies antivirus software over the weekend crippled some Windows XP PCs by mistakenly deleting a critical system file.

According to messages on AVG's support forums and its own support site, an update released late Saturday for the company's security software fingered the user32.dll file as a Trojan horse. As per the program's settings, the AVG software, including the newest version 8.0 and its predecessor 7.5, shut the .dll away in quarantine. The result: A crippled computer.

Flawed AVG antivirus update cripples Windows XP PCs