People install antivirus scanners on their systems to feel more secure, but a new update for the popular free AVG Virus Scanner had a very nasty bug that caused a lot of damage. The new virus definition file accidentally mistook user32.DLL, a critical Windows component, for a container for Trojan Horses PSW.Banker4.APSA or Generic9TBN and placed the file in quarantine:
When the scanner went active, it deleted this critical file, thinking it contained a virus, causing the system to crash. AVG recommended users whose definitions auto-updated delete their virus definition file and cancel any scans they have running.
If your computer is affected, it will either stop booting or go into an endless reboot loop. Vista users can breathe a sigh of relief -- so far that OS has remained relatively unaffected. Windows XP users, however must now exercise extreme caution, or risk having to carry out a bothersome repair process.
Both AVG 7.5 and AVG 8.0 were affected by the erroneous definition file. The file has since been update to remove the error.
For affected users, you can either reinstall Windows or repair it with a Windows disk. A third option is to use a boot disc, such as the Ultimate Boot CD (ISO) and then grab the files you need from the "C:WindowsSystem32dllcache" directory.