Have you had a look in the avast help file ?
Press F1 when the UI is open, there is a section on Boot-Time scan in the Scanning for viruses heading.
The rootkit scan is specific to finding rootkits (nothing else) and runs 8 minutes after avast starts.
There is a special rootkit log it is here, C:\Documents and Settings\All Users\Application Data\Alwil Software\Avast5\log\aswAr.log (for XP users, for other OSes you will need to search).