An update released on sunday for AVG Free led to the crash of many Windows XP pc's. In short, AVG sees the file user32.dll as a virus, when the user removes the file, Windows XP will no longer boot, safe mode does not work either.

Users with the problem can do a file restore trough the repare console on the Windows XP cd.

http://freeforum.avg.com/read.php?7,155461#msg-155501