Many users have reported a system crash resulting from an AVG update published on December 1. The update causes the computer to continually reboot and even booting into Safe Mode will not help. The solution to the AVG crash is to boot from a rescue CD and rename the AVG folder, after which the system should boot normally. (For step-by-step instructions, see How to Disable AVG When It Crashes Your Computer).
According to AVG, the problem mostly impacts x64 users of Windows 7 - though a search of forums indicates other operating systems may have been impacted as well.
AVG claims the problem resulted from improper inclusion of some untested files in the 12-1 update package. This does bring concerns for chain of custody to mind - hopefully something they will take steps to rectify in forthcoming releases.