The behaviour shield is a bit of a mystery! There's quite a few questions about it on the Avast forums and no real answers. I think the guys on there have been waiting for months for a 'promised' explanation from an Avast official.
Found out a few things tho. This is a quote from an interview with a Avast official ....
"The Behavior Shield that we shipped in version 5.0 is a new component that is going to be further developed moving forward. For example, in version 5.1, we will be adding more sensors that will allow for even finer-grain filtering.
For now, the Behavior Shield is focused on exploits coming via typical mechanisms (browser, PDF reader, and flash vulnerabilities, for example). It also closely monitors all kernel-mode code (drivers) loaded into the operating system, and is able to detect zero-day rootkits.
So whilst it's new and not operating at it's fullest potential, it still is an important part of Avast!
It's not supposed to use hardly any system resources, but a few people have had problems with it conflicting with other software. One person had the problem of it conflicting with the 'Payed' version of Malwarebytes. And thats interesting because my set up is the same as yours, Avast(free), Comodo firewall(free) but i have the Free version of Malwarebytes, and i have no problems with conflicts.
Its worth checking to see if there's any information in the Behaviour Shield report( at the bottom of the Behaviour shield page) in the avast user interface.
You could try to post a topic in the Avast forum http://forum.avast.com/index.php?board=2.0
Or you could just install a different free antivirus...... Avira is good
Edited by 666philb, 18 May 2010 - 01:06 PM.