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.
Tanked my xp64 system. Had to reinstall xp64 then painfully update via MS... didn't lose any data, but had to reinstall all my programs (which require Registry). After 3 days I'm about 95% restored on this machine. Absolute pain in the you know what. Immediately deleted AVG from 17 owned systems and sent an email to 150 clients running 64bit to remove the program and replace with either MS Essentials (won't work with xp64) or Avast.