This is a pretty old location - but I think it is still valid - I haven't done a boot scan for some considerable time so I don't have the aswboot.txt file to confirm.
Look in the C:\Documents and Settings\All Users\Application Data\Avast Software\Avast\report\aswBoot.txt file (XP location)
C:\ProgramData\Avast Software\Avast\report\aswBoot.txt (Vista, Win7 or later OS location), check this file using notepad for info on the scan/detections, etc.