It happened to me too. My wife's laptop ran Avast free on Windows 7 home premium 64bit, and a couple days ago EVERYTHING stopped working. No programs would startup, and I am unable to uninstall anything. Fortunately it's only a couple months old and I have the restore DVD's. It's fixed now, but, without knowing it was AVAST that caused the problem, I started up my desktop computer this morning. I was greeted with a promp to update the AVAST program and did so. Now my desktop, that's run great for over a year is hosed. The same problems with it as the laptop had. Unfortunately, I have no backup, and am struggling to find a way to back out the update. Since even AVAST won't start, and won't uninstall, I am in a quandry.
I had recommended AVAST to a lot of friends in the past year or two. Now I am trying to track them all down to warn them NOT to update the program. I hope AVAST has some kind of a fix for this as I can't stand to loose my desktop computer.