If you haven’t already downloaded the latest mandatory AVG 2011 Free software update, it may be best not to if you want your computer to turn on again. The December 1 update is causing 64-bit PCs running Windows 7 to totally breakdown.
The problem appears to lie in the 271.1.1/3292 (432/3292) database update, which requires PCs that run the update to restart. Affected PCs will throw up a c0000135 error upon restarting, and fail to boot.
AVG have since pulled the update so no more computers suffer, but if your computer got caught then all is not lost. AVG have released a handy set of instructions to follow, detailing how to disable the software if it bricks your system.
AVG Forum users have also come up with a few tricks to forcing the system to boot, such as pressing F8 and removing the AVG update. All methods will need the Repair CD in order to re-install the software.
AVG Free Update Bricks 64-Bit Windows 7 PCs - PCWorld Business Center