The update to the 2011 version of AVG antivirus will not allow your system to start.
From what I understand, this affects all Windows operating systems (xp, Vista, 7) and affects both the free, and paid versions of AVG.
If you have AVG, either uninstall it or set the automatic updates within AVG to notify but do not install. This problem has not been fixed by AVG. Their forums are full of customer complaints because of this. They claim that the issue is caused by a Zone Alarm toolbar bug, but I do not have Zone Alarm and have never had Zone Alarm on my computer. From reading the error report, the issue is with AVG renaming or deleting a system file and not replacing it upon restart. This error has ruined my opinion of AVG. They should know better. I'm not going to use or recommend their software any longer. I'm not telling you to boycott AVG, but I'm just sharing my experience.
Luckily, I had a complete backup from October, so I was able to restore everything.