As the Security and the
Net blog reports, a recent definition update for the popular AVG Free 7.5
and 8.0 anti-virus apps mistakenly tagged a Windows system file and told users
to delete it: "An update for the AVG virus scanner released yesterday contained
an incorrect virus signature, which led it to think user32.dll contained the
Trojan Horses PSW.Banker4.APSA or Generic9TBN. AVG then recommended deleting
this file; this causes the affected systems to either stop booting or go into a
continuous reboot cycle. So far, the problem only appears to affect Windows XP,
but there is no guarantee that other versions of Windows don't have the same
issue."
According to this post on the official forums, AVG has acknowledged the
problem and "immediately" posted a new definition update without the false
positive. If it's already too late for you, the post suggests either booting in
safe mode or using the Windows XP installation CD's recovery console to copy
user32.dll from C:\Windows\System32\dllcache to C:\Windows\System32.