On one very tiring day I decided I wanted to see that stupid useless video someone had sent me, and I updated Flash to see it. It failed and I thought no more of it.

Happily, the worm detected Avast and shut it down regularly and that's how I 4h later found out I had behaved like a regular user instead of a power one.

AV helps: 1) People do stupid things 2) defense in depth