"One popular free antivirus scanner recently created a world of woe for its users. The world of computer security can be a scary place for friends and foes alike.  This weekend users' found their AVG software updated with a new virus definition file.  Then they quickly found their computers crashing. 

What was discovered was that the new virus definition file mistook user32.dll, a critical Windows component, for a container for the Trojan Horses PSW.Banker4.APSA or Generic9TBN.  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:\Windows\System32\dllcache" directory.

With 80 million total users worldwide, thanks in part to AVG's free version, this error is obviously significant to many.  So far AVG has not issued a formal statement about the problem, although there are posts on their discussion board about it, to which they have responded informally."