This is an old post but...
I just can't let the statement about viruses stomping Avast go!
I've been intensively testing many of the popular AV for the last three years, and the only stomping I see is Avast kicking butt on the maleware.
It is true that Avast will let a lot of malware lay dormant; but that is because the malware coders are getting smart, and they program their malware to be firewall,anti-virus, and network aware; among several other things Michael points out.
So in my experience Avast will kick @ss once the malware makes a move and not before. It has been very successful and I've not had to do scans with it, unless I just want to test it's performance.
CCleaner usually removes anything left in the download files that is laying dormant, so I never find anything, even if I double check with an online scanner, or NIS2010, NOD32, or Prevx.
