I am having the exact same problem as the user in this thread:
http://forum.avast.com/index.php?topic=53548.0When I have the behavior shield on in Avast and download anything (whether it be a webpage in my browser or a large file) my computer will start to lag. Any streaming video will noticeably skip and lag. My mouse cursor will skip across the screen. Upon opening process explorer I find that about a third of my CPU is being used for "Deferred Procedure Calls". If I turn off the behavior shield or the download finishes my computer stops lagging and returns to normal.
I am using Windows 7 Ultimate 32 bit Edition. Once again, this only happens when my computer is downloading something (including web pages, pictures, anything that downloads onto the computer) and when the behavior shield is on.