There are a few of these threads all talking about Chrome installing without permission. This thread is the freshest of the bunch that I could find, so I'm posting here.
Here is one such older thread from over a year ago. This has been an ongoing issue for over a year and still has not been resolved. I can no longer recommend Avast to my friends and coworkers and will not be installing it on their machines anymore. I'm not going to recommend software to people that is going to auto-install another web browser. Even if it's a fluke and doesn't happen all the time, the fact that it's been an issue for over a year and Avast still can't figure it out causes me to lose faith in the company. Here is my specific experience with the issue that just happened a few minutes ago. I'll go ahead and copy and answer the questions from the other thread that I linked too.
My current updated Avast version is 8.0.1497. I don't know what version I had before, but it was most likely whatever version came before 8.0.1497 as I typically update within a few days of an update being made available. I seriously doubt I was multiple versions behind. Anyway, I updated Avast, and it asked for a reboot. I told it no. I went ahead and installed 8 Windows updates too. That got me up to date there as well except that I decided not to install IE 10 yet. I figured I'd install it after the reboot. For Avast update settings, I have it set to auto update the engine and virus definitions, but for PROGRAM updates it is set to ASK, which it did. After the Windows updates installed, I rebooted. I logged in with my password, and while things were loading, an Avast screen popped up with a FINISH button and two check boxes. One check box was for installing Chrome. I was not able to read the other box because the window disappeared before I could finish reading it and Chrome was subsequently installed. I assume the other box was for making Chrome the default browser, which also happened. So there was an option to uncheck the boxes, but the window closed before I had time to uncheck the boxes and I did NOT press the finish button. This is the exact behavior of option 2 in the older thread I linked from July 2012. Obviously Avast never resolved it. Here are my answers to those questions from that thread.
How can i help to get this problem solved?
Ranting and threatening won't help, but answering few short questions may help us...
1. What avast! edition are you using (ie Free, Pro or AIS)?
2. When did this happen (during update or new installation)?
3. What Windows version are you using (ie Win7 SP1 64bit)?
4. Which was your default web browser at the time this happened?
5. Have you ever installed Google Chrome yourself in the past (and then removed it)?
6. How did this happen (dialog quickly disappeared, missing checkboxes etc)?
7. When did this happen (right after update, after update reboot, both)?
8. If you think any other info might be relevant or helpful, please post it.
1. Free
2. Update
3. Win7 SP1 64bit
4. Firefox 23.0.1
5. The only time Chrome has been installed is when it was previously installed by Avast and I removed it. This isn't the first time it has happened, but it's been a while.
6. Dialog quickly disappeared and the boxes are OPT OUT instead of OPT IN. Not enough time to uncheck the boxes.
7. Happened right after update and reboot.
8. Nothing I can think of at the moment.