Earlier this week, users of AVG’s virus scanner accidentally got asked to ‘remove’ user32.dll, a core system file for the Windows operating system. AVG mistakenly thought that user32.dll contained one of two Trojan horses – PSW.Banker4.APSA or Generic9TBN.

Therefore users were requested to delete the file. However, the problem only affects users of AVG 8 products running the Dutch, French, Italian, Portuguese, and Spanish language versions of Windows XP. Users who followed AVG's recommendation to delete the file were left with computers that either would not boot up or continually rebooted. The AVG Free Forum filled with users looking for advice.

AVG on Sunday released a further update that did not incorrectly identify the core Windows file as malware.

Both AVG 7.5 and 8.0 were affected by the update and a revised signature database has just been published that corrects this issue. The company reported that anyone who has removed the user32.dll can either boot from their original Windows CD and choose the repair option, or use another CD to boot from and restore the file from C:\Windows\System32\dllcache.

“Unfortunately, the previous virus database might have detected the mentioned virus on legitimate files. We can confirm that it was a false alarm. We have immediately released a new virus update (270.9.0/1778) that removes the false positive detection on this file. Please update your AVG and check your files again. We are sorry for the inconvenience and thank you for your help,” AVG responded.

This is not the first such false positive AVG has had to fix. Its software wrongly identified CheckPoint’s Zone Alarm as a Trojan less than a month ago. Therefore AVG’s credibility may become more unreliable as we speak.