Hello,
For two times Avast hanged completely my computer, I still could move the mouse but all applications were unresponsive. I could heard some hard disk noise when that happened (no hardware problem).
Killing those applications would not solve the issue so I had to forze a restart.
I decided to uninstall Avast until this gets solved, I don't know how to reproduce the problem.
I'm using Mac OS X 10.6.8.
Thanks!