I have read of others problems with Behavior Shield and I can relate. I have been a user of Avast for about 5 years. Recently the one year expired and when I needed to refresh my install, a new version was installed.
After that my system was behaving very differently. The computer was responding so slowly I thought I might have picked up a virus. One other odd behavior: My A: drive kept flashing even though nothing was in the drive.
Anyhow after doing a Google search on “Avast A: drive keeps flashing” I came on this forum message: http://forum.avast.com/index.php?topic=69367.0
Once I uninstalled Behavior Shield, my problems went away and my PC is running fine as before.