Windows 7 64-bit users who have not downloaded AVG 2011 antivirus update should know that they had a near miss from completely crashing their Windows installations. Non 64-bit users also need to be careful, because the actual details on other Windows platforms are sketchy at the moment. The update prevents 64-bit versions from booting up at all. The critical bug stems from 271.1.1/3292 (432/3292) update, which comes with a mandatory restart. Users who install the update are shown a 'c0000135 error' on the subsequent restart, which brings about a total boot failure.

AVG hasn't detailed the exact cause of the problem, but such issues happen when the anti-virus algorithm wrongly identifies a critical Windows process as malicious and quarantines/deletes it. The update has been rescinded from AVG servers, so there is not further risk of harming your computer. But those who have managed to have crash their PCs should head over to this blog post from AVG that apologises for the mistake and provides detailed instructions to fix the same.