I found out that the problem with avast! and Opera was due to an old Opera skin, so I switched to the default skin and to avast!.
http://forum.avast.com/index.php?topic=79331.msg652893#msg652893
Now with avast! Free after a while my computer freezes.
I have to do a forced restart or start up.
I went back to MSE. No problems.
I reloaded my os before installing avast!.