That must be a Vista/Windows 7 path, since I am running XP Home, I found the file at:

C:\Documents and Settings\All Users\Application Data\Alwil Software\Avast5\report\BehaviorShield.txt

Both the BehaviorShield and FileSystemShield reports end with:

*
* avast! Real-time Shield Scan Report
* This file is generated automatically
*
* Started on: Sunday, August 28, 2011 8:08:08 AM
*

8/28/2011 8:08:21 AM	Modification of: \REGISTRY\USER\S-1-5-21-1844237615-2147015713-682003330-1004\Control Panel\Desktop\TileWallpaper
    By:  C:\Program Files\XemiComputers\Active Desktop Calendar\ADC.exe
    Via: C:\Program Files\XemiComputers\Active Desktop Calendar\ADC.exe
         -> Action allowed

*
* Shield stopped: Sunday, August 28, 2011 5:17:39 PM
* Run-time was 9 hour(s), 9 minute(s), 31 second(s)
*

*
* avast! Real-time Shield Scan Report
* This file is generated automatically
*
* Started on: Sunday, August 28, 2011 5:27:39 PM
*

The last two entries show where I stopped the shields for 10 minutes to allow Steam to update the game after it had already failed even with the exclusion in place.

Unrelated: How do I contact the admins of this forum, I want to change my signature but that feature appears to be hidden, under Profile all I get to change is the Account Related Settings.