Hi all.
I have this problem too.
99% of the time I only have msn messenger, notepad and firefox up.
When the freezing occurs, the avast icon usually spins slower than normal.
Usually most of the time I have 4-5 firefox windows opens and if I shut them all off and reopen firefox, the problem goes away. But also notice it seems something in one of the firefox displayed windows is causing it as a couple of times, if I shut down one of them while leaving the rest up, it the problem also goes away.